LIBRARY 

THE  UNIVERSITY 
OF  CALIFORNIA 

SANTA  BARBARA 

PRESENTED  BY 
C.   G.    NIELSEN 


FIG.  A. — OSCILLATORY  DISCHARGES. 
S',  exciting  oscillatory  discharges ; 
S,  in  a  neighbouring  conductor. 


PlG.      B. — NON-OSCILLATORY     SPARKS 

S',  exciting  oscillatory  movements ; 
S",  in  a  neighbouring  conductor. 


WHAT    IS 
ELECTRICITY? 


BY 

JOHN   TROWBRIDGE,   S.  D. 

RUMFORD    PROFESSOR    AND    LECTURER    ON    THE    APPLICATIONS    OF 

SCIENCE    TO    THE    USEFUL    ARTS,    HARVARD    UNIVERSITY, 
AND    DIRECTOR    OF    THE   JEFFERSON    PHYSICAL    LABORATORY 


ILLUSTRA  TED 


NEW     YORK 
D.    APPLETON    AND    COMPANY 


COPYRIGHT,  1896, 
BY  D.  APPLETON  AND  COMPANY. 


PREFACE. 


I  AM  often  asked  the  question,  "  What  is  electricity  ? " 
and  I  have  endeavoured  in  this  book  to  give  in  a  pop- 
ular manner  the  present  views  of  scientific  men  in  re- 
gard to  this  question.  According  to  modern  ideas,  the 
continuance  of  all  life  on  the  earth  is  due  to  the  elec- 
trical energy  which  we  receive  from  the  sun ;  and  phys- 
ics, in  general,  can  be  defined  as  that  subject  which 
treats  of  the  transformations  of  energy.  I  have  there- 
fore presented  the  varied  phenomena  of  electricity  in 
such  a  manner  that  the  reader  can  perceive  the  physicist's 
reasons  for  supposing  that  all  space  is  filled  with  a  me- 
dium which  transmits  electro-magnetic  waves  to  us  from 
the  sun. 

In  Tyndall's  Heat  as  a  Mode  of  Motion,  Tait's  Ee- 
cent  Advances  in  Physical  Science,  and  Stewart's  Con- 
servation of  Energy,  the  relations  between  work  done 
and  heat  produced  have  been  treated  in  a  popular  man- 
ner ;  but  I  am  not  acquainted  with  any  treatise  in  which 
Maxwell's  great  generalization,  entitled  the  Electro- 
Magnetic  Theory  of  Light,  has  been  made  the  basis  of 


vi  WHAT  IS  ELECTRICITY? 

a  popular  treatment.  The  wide-embracing  nature  of 
this  theory  can  be  seen  when  we  realize  that,  according 
to  it,  all  phenomena  of  light,  heat  as  well  as  those  of 
electricity,  are  manifestations  of  electrical  energy. 

I  have  used  in  this  treatise  various  popular  lectures 
which  I  have  delivered,  and  certain  articles  which  I 
have  published  in  the  Chautauquan,  the  Popular  Sci- 
ence Monthly,  the  American  Journal  of  Science,  and 
the  London  Philosophical  Magazine. 

I  realize  fully  the  difficulty  of  stating  accurately  in 
a  popular  exposition  what  is  more  definitely  expressed 
in  mathematical  language.  If  I  have  succeeded  in  giv- 
ing the  general  reader  an  idea  of  the  present  direction 
of  investigation  in  the  science  of  electricity  I  shall  con- 
sider myself  fortunate. 


CONTENTS. 


CHAPTER  PAGE 

I.— THE    STANDPOINT    OP    PHYSICISTS     .                                    .            .  1 

II. — MEASUREMENTS  IN  ELECTRICITY  .        .        .        .        .  11 

III.— MAGNETISM 25 

IV.— THE    ELECTRIC   CURRENT 45 

V. — FLOW  OF  ELECTRICITY  IN  THE  EARTH      ....  57 

VI.— THE  VOLTAIC  CELL .64 

VTL— THE  GALVANOMETER 86 

VIII.— THE   DYNAMO  MACHINE '.            .  94 

IX.— SOURCES  OF  ELECTRIC  POWER 105 

X. — TRANSFORMATIONS  OF  ENERGY 121 

XL— ALTERNATING  CURRENTS 141 

XI  I.— TRANSMISSION  OF  POWER  BY  ELECTRICITY   .        .        .  152 

XIII. — SELF-INDUCTION 164 

XIV.— THE  LEYDEN  JAR 171 

XV.- — STEP-UP  TRANSFORMERS 187 

XVI.— LIGHTNING 198 

XVII.— WAVE  MOTION 215 

XVIII.— ELECTRIC  WAVES 239 

XIX.— THE   ELECTRO-MAGNETIC     THEORY    OF    LIGHT   AND   THE 

ETHER        .           . 264 

XX.— THE  X  RAYS 287 

XXL— THE  SUN 298 

XXII.— WHAT  is  ELECTRICITY?        ......  305 

vii 


WHAT   IS   ELECTRICITY? 


CHAPTEK  I. 

THE    STANDPOINT   OF   PHYSICISTS    IN   REGARD   TO   THE 

QUESTION,  "WHAT  is  ELECTRICITY?" 

THE  question,  "  What  is  electricity  ? "  is  often  asked 
as  if  a  short  and  lucid  answer  could  be  given  which  a 
liberally  educated  person  could  comprehend.  In  order 
to  understand  the  grounds  upon  which  a  natural  phi- 
losopher bases  his  attempts  to  answer  this  question  one 
must  consider  the  entire  field  of  activity  in  which  we 
find  ourselves — a  field  which  we  shall  see  is  now  be- 
lieved to  be  due  to  the  electrical  energy  of  the  sun. 
The  subject  of  physics  can  be  said  to  be  the  study  of 
the  transformations  of  energy,  and  it  is  the  object  of 
this  treatise  to  describe  in  a  popular  manner  how  a 
great  intellectual  movement  which  is  now  going  on 
silently  and  steadily,  constantly  pushing  back  the  limit 
of  our  ignorance  and  occasionally  lifting  its  veil,  has 
taken  the  place  of  unsystematic  investigation  and  sterile 
philosophic  vagaries. 

Indeed,  the  characteristic  of  physical  science  to-day 
is  its  reliance  upon  patient  observation  and  the  study  of 
the  transformation  of  electricity  into  light  and  heat  or 
the  transformation  of  heat  into  electricity.  A  well- 


2  WHAT  IS  ELECTRICITY! 

trained  physicist  listens  with  as  much  intolerance  to 
the  speculations  of  a  philosopher  on  the  origin  of  force 
as  Moltke  would  have  listened  to  the  philosophical 
faculty  of  the  University  of  Berlin  on  the  origin 
of  war. 

The  great  modern  intellectual  movement  in  physical 
science  resides  in  the  abandonment  of  mere  speculation, 
and  the  substitution  for  it  of  the  study  of  detail  and 
the  investigation  of  the  economy  of  Nature  in  the  trans- 
formations of  the  store  of  energy  which  has  been  vouch- 
safed to  the  world.  Every  university  in  the  world  now 
has  its  systematic  laboratories ;  and  the  methods  of  pa- 
tient investigation  which  characterize  the  laboratory 
study  of  science  are  slowly  creeping  into  the  study  of 
other  subjects,  notably  that  of  law,  and  are  destined, 
we  believe,  to  be  adopted  in  all  subjects. 

Lord  Salisbury,  in  an  address  before  the  British 
Association  for  the  Advancement  of  Science  at  Oxford, 
1894,  said :  "  Science  in  the  universities  for  many  gen- 
erations bore  a  signification  different  from  that  which 
belongs  to  it  in  this  assembly.  It  represented  the 
knowledge  which  alone  in  the  Middle  Ages  was  thought 
worthy  of  the  name  of  science.  It  was  the  knowledge 
gained  not  by  external  observation,  but  by  mere  re- 
flection. The  student's  microscope  was  turned  inward 
upon  the  recesses  of  his  own  brain,  and  when  the  sup- 
ply of  facts  and  realities  failed,  as  it  very  speedily  did, 
the  scientific  imagination  was  not  wanting  to  furnish  to 
successive  generations  an  interminable  series  of  con- 
flicting speculations." 

The  chief  characteristic  of  this  marked  intellectual 
method,  the  most  noteworthy  movement  which  scien- 
tific education  has  seen,  is  the  accurate  study  of  the 
transformations  of  energy ;  for  we  perceive  that  there 


THE  STANDPOINT   OF  PHYSICISTS.  3 

is  much  to  occupy  the  investigator  in  this  subject  and 
that  constant  work  is  repaid  by  results,  whereas  philo- 
sophical speculations  upon  what  force  is  and  what  elec- 
tricity is  gives  us  no  vantage  ground  over  the  meta- 
physicians. 

Agnosticism  in  physical  science  is  a  hopeful  creed 
when  it  is  enlivened  by  a  quick  imagination,  which  is 
employed  by  the  laboratory  worker  to  suggest  clues 
to  follow  in  his  study  of  the  transformations  of 
energy. 

There  is  no  tendency  to  restrain  the  imagination  in 
this  attitude  of  scientific  agnosticism.  The  physicist  of 
to-day  has  his  ethers  and  his  atoms  just  as  the  ancient 
Greek  and  Eoman  philosophers  had  theirs,  and  he  pic- 
tures to  himself  invisible  motions  far  more  subtle  than 
entered  the  imagination  of  Aristotle  or  Democritus. 
The  natural  philosopher  of  to-day,  however,  differs  in 
this  essential  respect  from  the  ancient  philosopher: 
he  measures.  If  his  heat  measures  do  not  agree  with 
his  hypotheses  of  vortical  or  atomic  motions,  he  re- 
jects his  attractive  hypotheses  instead  of  hugging 
them. 

In  my  attempt,  therefore,  to  give  in  a  popular  man- 
ner the  speculations  of  physicists  upon  the  question, 
"  What  is  electricity  ? "  we  must  carefully  bear  hi  mind 
the  standpoint  of  investigators  and  the  way  in  which 
they  hold  their  hypotheses.  However  attractive  the 
hypotheses,  they  are  ruthlessly  abandoned  as  soon  as 
the  touchstone,  the  measurement  of  the  heat  equiva- 
lent of  the  motion,  is  not  satisfied  by  the  hypotheses. 
It  is  not  often  that  one  finds  an  intelligent  appreci- 
ation of  this  attitude  of  holding  hypotheses  in  suspense 
which  is  characteristic  of  the  best  minds  in  science  of 
to-day,  and  indeed  the  task  set  for  the  physicists  is  not 
2 


4  WHAT  IS  ELECTRICITY? 

fully  comprehended.  They  are  a  small  body  of  men  to 
whom  the  world  looks  especially  for  exact  information. 
By  means  of  patient  measurement  in  the  great  field  of 
the  transformations  of  energy  they  have  been  able  to 
supply  the  world  with  the  most  exact,  if  not  perfectly 
exact,  information  which  it  now  possesses.  Since  the 
subject  of  this  treatise  is  a  popular  presentation  of 
what  I  regard  as  the  real  subject  of  physics — the 
transformations  of  energy — and  of  the  greatest  gen- 
eralization in  that  subject  (Maxwell's  Theory  of  Elec- 
tro-magnetic Origin  of  Light  and  Heat),  I  can  not  do 
better  than  to  quote  his  words  in  regard  to  the  attitude 
of  mind  and  mental  characteristics  of  the  order  of  men 
whose  speculations  we  are  about  to  study : 

"  With  respect  to  the  '  material  sciences,'  they  appear 
to  me  to  be  the  appointed  road  to  all  scientific  truth, 
whether  metaphysical,  mental,  or  social.  The  knowl- 
edge which  exists  on  these  subjects  derives  a  great  part 
of  its  value  from  ideas  suggested  by  analogies  from  the 
material  sciences,  and  the  remaining  part,  though  valu- 
able and  important  to  mankind,  is  not  scientific,  but 
aphoristic.  The  chief  philosophical  value  of  physics  is 
that  it  gives  the  mind  something  distinct  to  lay  hold  of, 
which,  if  you  don't,  Nature  at  once  tells  you  you  are 
wrong.  Now,  every  stage  of  this  conquest  of  truth 
leaves  a  more  or  less  presentable  trace  on  the  memory, 
so  that  materials  are  furnished  here  more  than  anywhere 
else  for  the  investigation  of  the  great  question,  '  How 
does  knowledge  come  ? ' 

"  I  have  observed  that  the  practical  cultivators  of  sci- 
ence (e.  g.,  Sir  J.  Herschel,  Faraday,  Ampere,  Oersted, 
Newton,  Young,),  although  differing  excessively  in  turn 
of  mind,  have  all  a  distinctness  and  a  freedom  from  the 
tyranny  of  words  in  dealing  with  questions  of  order, 


THE  STANDPOINT  OF  PHYSICISTS.  5 

law,  etc.,  which  pure  speculators  and  literary  men  never 
attain."  * 

I  have  said  that  the  measurements  of  heat  produced 
by  motion  constitute  our  great  touchstone  in  testing 
the  truth  of  physical  hypotheses.  Tyndall's  treatise  on 
Heat  a  Mode  of  Motion  was  an  epoch-making  book 
in  popular  estimation.  It  certainly  brought  readers  to 
realization  of  the  great  truth  that  all  motion  has  its 
equivalent  in  heat.  The  treatise  was  called  Heat  con- 
sidered as  a  Mode  of  Motion.  However  much  we  may 
be  inclined  to  criticise  the  title — for  we  can  not  prop- 
erly class  the  motions  which  produce  heat  in  a  class  as 
modes — we  must  recognise  that  it  presented  certain  as- 
pects of  the  law  of  conservation  of  energy  in  a  singu- 
larly lucid  manner.  There  is  no  hint,  however,  in  the 
treatise  of  a  possible  relation  between  motion,  light,  heat, 
and  electricity.  The  lectures  which  form  the  basis  of 
Tyndall's  treatise  were  delivered  before  the  Royal  In- 
stitution in  1862,  and  in  the  index  to  the  volume  one 
finds  no  reference  to  Maxwell  and  his  great  electro- 
magnetic theory  of  light.  In  fact,  Maxwell  was  ready 
to  come  forward  with  his  great  generalization  at  the 
time  Tyndall  was  bringing  the  world  to  a  realizing 
sense  of  the  conservation  of  energy. 

Maxwell's  theory  that  light  and  heat  are  phenomena 
of  electro-magnetic  waves  which  come  to  us  from  the  sun 
is  now  the  greatest  generalization  in  physical  science,  and 
in  stating  it  Maxwell  lighted  a  torch  which  has  illumined 
many  hitherto  dark  regions.  It  is  safe  to  affirm  that 
the  entire  world  is  now  working  upon  this  great  hy- 
pothesis. According  to  the  electro-magnetic  theory  of 


*  Letter  to  R,  B.  Litchfield,  Life  of  James  Clerk  Maxwell  (Camp- 
bell and  Garnett)  p.  305. 


6  WHAT  IS  ELECTRICITY? 

light,  the  only  difference  between  light,  heat,  and  elec- 
tricity consists  in  the  length  of  waves  in  the  ether  of 
space.  The  sun  is  the  source  of  electro-magnetic 
waves,  and  the  earth  is  the  scene  of  transformations  of 
electric  energy.  A  piece  of  coal  burning  in  a  grate 
has  therefore  a  long  electro-magnetic  history.  It  owed 
its  origin  to  electro-magnetic  waves,  and  in  burning  it 
gives  out  again  electro-magnetic  waves,  of  which  we 
can  only  detect  the  light  and  heat  manifestations. 

The  Rumford  professorship  in  Harvard  University 
was  endowed  by  Count  Rumford  in  order  to  promote 
our  knowledge  of  light  and  heat ;  for  many  years  the 
lectures  given  under  the  endowment  were  devoted  ex- 
clusively to  the  subject  of  light  and  heat  and  the  con- 
servation of  energy — a  subject  to  which  Count  Rum- 
ford's  experiments  may  be  said  to  have  given  one  of 
the  primal  impulses.  The  lecturer  and  student  to-day, 
however,  is  compelled  to  approach  the  subject  of  light 
and  heat  through  the  broadest  study  of  electromag- 
netism. 

Tyndall,  I  have  said,  in  his  Heat  considered  as  a 
Mode  of  Motion,  has  happily  illustrated  the  great  doc- 
trine of  the  conservation  of  energy.  No  truth  is  better 
established  in  physics  than  that  of  the  equivalence  be- 
tween work  and  the  heat  produced,  and  the  modern 
developments  of  electricity  afford  means  for  richly  illus- 
trating it.  The  claims  of  the  great  founders  of  this 
hypothesis  have  been  set  forth  by  Tyndall  in  his  treatise, 
and  by  Tait  in  his  Recent  Advances  in  Physical  Science. 
My  object  in  this  treatise  is  to  call  attention  to  the  trans- 
formations of  energy  rather  than  to  discuss  the  claims 
of  priority ;  to  follow  the  protean  forms  under  which 
energy  manifests  itself,  rather  than  to  measure  the 
equivalence  of  these  forms  of  energy. 


THE  STANDPOINT  OF  PHYSICISTS.  f 

If  we  wish  at  this  stage  of  our  presentation  of  the 
subject  of  the  "transformation  of  energy  exultantly  to 
proclaim  the  results  of  the  methods  of  the  measure- 
ments of  heat  equivalents  of  motion,  we  can  point  to 
the  commercial  applications  of  electricity,  which  are  all 
based  upon  exact  experiments  on  the  equivalence  be- 
tween the  heat  produced  by  the  current  and  the  work 
done  by  the  steam  engine  which  produces  the  current. 

The  doctrine  of  the  conservation  of  energy  is  little 
more  than  a  hundred  years  old.  Count  Eumford  made 
his  celebrated  measurement  of  the  heat  produced  by 
the  boring  of  a  cannon  in  1789.  Yet  in  this  time  the 
world,  working  with  this  powerful  theory,  has  made 
greater  progress  than  it  did  with  its  physical  specula- 
tions during  the  comparatively  immense  historic  period 
which  preceded  1789. 

Not  only  in  the  commercial  world  do  we  find  that 
the  great  physical  doctrine  has  accomplished  great  re- 
sults. In  the  subject  of  medicine  the  doctrine  is  daily 
being  recognised ;  for  the  application  of  external  heat 
to  the  human  body  to  dimmish  the  effort  of  the  human 
organism  to  supply  heat,  or  to  supplement  this  effort 
in  very  young  children  or  in  very  old  people,  is  now 
clearly  understood.  The  death  of  very  old  persons 
at  night  is  often  due  to  the  want  of  heat.  They  can  be 
said  to  freeze  to  death.  Yery  young  children  often 
perish  because  sufficient  heat  is  not  supplied  to  them. 
It  seems,  therefore,  to  be  both  physically  and  physi- 
ologically unphilosophical  to  expose  the  limbs  of  young 
children  to  the  cold  air.  In  hospitals,  during  severe 
operations,  the  patient  is  placed  upon  a,  warm  table  and 
is  afterward  surrounded  with  hot-water  bags  or  similar 
contrivances  to  supply  heat.  This  external  heat  facili- 
tates the  various  transformations  of  energy  which  are 


3  WHAT  IS  ELECTRICITY? 

going  on  in  the  human  organism,  and  too  great  a  de- 
mand is  not  made  upon  the  internal  mechanism  to  sup- 
ply this  heat. 

The  study  of  the  transformations  of  energy  in  the 
human  body  is  a  far  more  difficult  one  than  the  study 
of  the  duty  of  an  ordinary  steam  engine,  for  the  trans- 
formations are  more  numerous  and  subtle.  In  the 
main,  however,  there  is  a  close  relation  between  the 
amount  of  food  consumed  and  the  work  that  a  man 
can  do. 

The  most  powerful  instrument  for  studying  the 
transformation  of  energy  is  still  the  steam  engine.  By 
means  of  this  great  invention  of  Watt  we  obtain  our 
electrical  currents ;  and  when  it  is  said  that  electricity 
will  some  day  supersede  steam,  we  can  assert  with  a 
similar  show  of  reason  that  flying  may  some  day  take 
the  place  of  locomotion  on  the  common  road.  At 
present  there  is  no  way  of  accomplishing  the  wished-f  or 
result.  Steam  produces  electricity,  and  it  is  the  most 
economical  agent  for  producing  it. 

It  is  interesting  to  reflect  that  steam  is  produced  by 
the  combustion  of  a  past  vegetation.  The  great  tree 
ferns  of  the  carboniferous  age  required  for  their  com- 
plex life  the  transformation  of  the  sun's  energy  into 
chlorophyl  and  the  tissues  of  these  strange  growths. 
This  varied  transformation  is  again  made  manifest  in 
the  wonderful  action  of  coal-tar  dyes  in  modern  pho- 
tography. Thus  the  original  action  of  electro-magnetic 
waves  shown  in  complex  growths  of  vegetation,  buried 
in  the  earth  for  ages,  becomes  evident  again  in  a  grand 
series  of  transformations.  The  burning  of  a  fossilized 
tree  trunk  produces  steam,  steam  is  converted  into 
motion,  motion  into  electricity,  and  electricity  into  heat 
and  light. 


THE  STANDPOINT  OF  PHYSICISTS.  9 

I  have  said  that  steam  is  at  present  our  cheapest 
source  of  electricity.  "With  the  best  engines  it  requires 
little  more  than  a  pound  of  coal  to  produce  a  horse 
power.  This  is  a  cheaper  horse  power  than  we  can 
produce  by  water  power,  for  the  regulation  and  control 
of  the  supply  of  water  is  more  difficult  than  that  of  a 
supply  of  steam.  We  thus  see  in  New  England  the 
manufacturing  towns  of  New  Bedford  and  Fall  River, 
where  there  is  no  water  power,  competing  in  number 
of  spindles  with  Lowell  and  Lawrence,  on  the  banks  of 
the  Merrimac  River. 

In  studying  the  steps  by  means  of  which  Faraday 
and  Joseph  Henry  detected  the  small  indications  of  a 
force  which  the  steam  engine  has  exalted  into  a  mighty 
one,  we  are  reminded  of  the  slight  rub  which  Aladdin 
gave  to  the  lamp  which  was  sufficient  to  summon  a 
genius  who  could  perf orm  any  tasks,  from  the  most  deli- 
cate one  to  the  most  tremendous.  The  feeble  manifes- 
tation discovered  by  Faraday  and  Henry  of  what  can 
be  made  a  great  force  is  called  the  force  of  magnetic 
induction.  It  may  be  said  that  it  laid  perdu  with  the 
possibility  of  being  called  into  action  by  any  one  who 
possessed  a  coil  of  wire  and  a  magnet.  The  reason 
that  it  remained  undiscovered  so  long  was  due  to  the 
difficulty  of  obtaining  well-insulated  copper  wire,  and 
to  the  want  of  a  sufficiently  sensitive  instrument  to  de- 
tect it. 

The  story  of  the  discovery  of  magnetic  induction  by 
Faraday  and  Henry  is  most  instructive,  for  it  shows 
how  an  apparently  slight  and  unimportant  manifesta- 
tion of  energy  can  be  exalted  by  proper  means  into  a 
tremendous  one.  Faraday  remarked,  after  detailing  his 
experiments  on  magnetic  induction :  "  The  various  ex- 
periments of  this  section  prove,  I  think,  most  completely 


10  WHAT  IS  ELECTRICITY? 

the  production  of  electricity  from  ordinary  magnetism. 
That  its  intensity  should  be  very  feeble  and  quantity 
small  can  not  be  considered  wonderful,  when  it  is  re- 
memhered  that,  like  thermoelectricity,  it  is  evolved  en- 
tirely within  the  substance  of  rnetals  retaining  all  their 
conducting  power" 

The  steam  engine  has  exalted  this  apparently  feeble 
effect  discovered  by  Faraday  into  a  power  which  is  only 
limited  by  that  of  the  steam  engine  or  the  water  power 
which  we  employ. 


CIIAPTEE  II. 

MEASTJKEMENTS    IN   ELECTRICITY. 

WE  have  said  that  the  subject  of  electricity  has 
made  its  great  strides  during  the  past  fifty  years  by  the 
intelligent  application  of  the  doctrine  of  the  conserva- 
tion of  energy  to  it,  and  by  the  measurements  of  the 
heat  equivalents  of  various  forms  of  motion.  In  a 
popular  treatise  it  is  difficult  to  present  the  more  or  less 
dry  details  of  exact  measurements.  I  wish  here  merely 
to  emphasize  the  fact  that  the  mysterious  force  of 
gravitation — perhaps  more  mysterious  in  its  manifesta- 
tion than  any  electrical  or  magnetic  force — is  used  to 
measure  all  our  electrical  manifestations. 

It  has  been  suggested,  it  is  true,  that  the  time  of 
vibration  of  a  hydrogen  molecule  at  a  definite  tempera- 
ture and  pressure  would  be  an  unalterable  standard  of 
time,  and  that  the  wave  length  in  vacuum  of  sodium 
light  would  form  a  standard  of  length  independent  of 
any  change  in  the  force  of  gravitation  and  in  the  di- 
mensions of  the  earth.  Maxwell  remarks  in  regard  to 
this  last  suggestion  that  "  it  should  be  adopted  by  those 
who  expect  their  writings  to  be  more  permanent  than 
that  body." 

"We  have  what  is  called  an  absolute  system  of  elec- 
trical measurements — absolute  in  the  sense  that  every- 
thing is  referred  to  acceleration  of  a  body  falling  to  the 


12  WHAT  IS  ELECTRICITY? 

earth  at  a  definite  place  through  a  certain  space  in  a 
definite  time.  Absolute  also  in  the  sense  that  we  do 
not  deal  with  merely  ratios. 

We  measure  the  eificiency  of  a  dynamo  which  is 
producing  the  currents  of  electricity  which  are  used  in 
lighting  our  houses  and  propelling  our  cars  by  means 
of  the  mechanical  equivalent  of  heat,  which  states  that 
the  work  done  in  raising  one  pound  of  water  one  degree 
in  temperature  on  the  Fahrenheit  scale  is  equivalent  to 
raising  772  pounds  one  foot  high  against  the  attraction 
of  gravitation.  Our  ultimate  appeal,  therefore,  on  the 
subject  of  the  transformation  of  energy  upon  which  we 
are  entering  is  to  gravitation,  and  it  will  be  interesting 
at  the  opening  stage  of  our  study  of  electro-magnetism 
to  consider  gravitation  as  our  measurer  of  electrical 
energy. 

In  general  terms,  we  measure  the  quantity  of  elec- 
tricity which  is  delivered  along  a  wire  by  the  current 
which  is  flowing  multiplied  by  the  time  during  which 
it  flows.  Now,  the  time  is  measured  by  a  pendulum 
which  depends  for  its  action  upon  the  force  of  gravita- 
tion. Our  standard  for  the  measure  of  time  depends 
upon  the  pendulum,  and  this  in  turn  upon  the  time  of 
rotation  of  the  earth.  It  is  true  that  we  may  depend 
upon  a  tuning  fork  for  the  estimation  of  very  short 
intervals  of  time,  but  the  fork  in  turn  is  standardized 
by  a  second  pendulum. 

By  means  of  measurements  based  upon  the  law  of 
gravitation  scientific  men  use  constantly  the  only  known 
universal  language — that  of  absolute  measurements. 
When  an  English-speaking  physicist  expresses  the  re- 
sults of  his  measurements  in  centimetres,  in  grammes, 
and  in  seconds,  he  knows  that  he  will  be  understood  by 
a  German,  a  Russian,  a  Frenchman,  or  an  Italian ;  and 


MEASUREMENTS  IN  ELECTRICITY.  13 

it  can  be  said  that  no  other  realm  of  human  endeavour 
lias  such  a  universal  language.  It  is  curious  to  note  the 
disposition  of  the  human  mind  to  dwell  upon  the  mys- 
teries of  electricity  and  magnetism,  and  to  totally  ignore 
the  greater  mystery  of  gravitation.  We  are  beginning 
to  have  an  inkling  of  the  relations  of  electricity  and 
magnetism  to  light  and  heat  and  to  motion.  Every  day 
fresh  evidences  of  the  laws  of  the  transformations  of 
energy  increases  our  knowledge  upon  electricity,  but 
we  are  absolutely  ignorant  of  the  relationship  of  gravi- 
tation to  the  subject  of  electricity  and  magnetism,  light, 
heat,  and  motion.  Gravitating  force,  by  means  of 
which  we  measure  electricity,  is  perhaps  the  greatest 
mystery  in  the  subject  of  physical  science,  and  its  mani- 
festation is  so  omnipresent,  so  silent  and  unsensational, 
that  our  mind  rarely  dwells  upon  its  mysterious  action. 
The  work  we  have  to  do  to  overcome  the  force  of 
gravitation  is  our  measure  of  it.  When  large  masses 
are  lifted  against  this  force  we  become  sensible  of  its 
potency.  Yet  the  force  of  attraction  between  two  small 
bodies,  such  as  two  cannon  balls,  is  extremely  difficult  to 
detect  and  to  measure.  The  direct  determination  of 
the  attraction  between  two  masses  by  means  of  the  com- 
mon balance  is  the  simplest  way  of  obtaining  a  realizing 
sense  of  the  magnitude  of  this  force.  Prof.  Poynting, 
by  many  refinements,  has  made  it  also  one  of  accuracy. 
The  method  he  at  first  adopted  was  to  suspend  a  mass 
from  one  arm  of  a  balance  by  a  long  wire  and  counter- 
poise it  in  the  other  pan  ;  then  by  bringing  under  it  a 
known  mass,  its  weight  would  be  slightly  increased  by 
the  attraction  of  this  mass.  Prof.  Poynting  showed 
that  this  increase  in  weight  would  be  the  quantity  sought 
if  the  attracting  mass  had  no  appreciable  effect  before 
its  introduction  beneath  the  hanging  mass,  and  if,  when 


14  WHAT  IS  ELECTRICITY? 

beneath  it,  the  effect  on  the  balance  could  be  neglected. 
It  was  found  that  a  mass  of  453  grammes  of  lead  hung  on 
one  arm  of  a  chemical  balance  by  a  wire,  and  attracted  by 
a  mass  of  154  kilogrammes  of  lead,  showed  an  apparent 
increase  of  about  0*01  milligramme.  We  perceive  from 
this  how  small  the  force  is  which  is  to  be  measured,  and 
in  order  to  determine  it  with  accuracy  Prof.  Poynting 
adopted  a  differential  method,  which  consisted  in  sus- 
pending an  attracting  mass  from  each  arm  of  a  balance 
instead  of  from  one  arm,  and  bringing  another  attract- 
ing mass  first  under  one  suspended  mass  and  then  under 
the  other.  By  this  differential  method  certain  errors 
were  eliminated.  It  was  found  th.at  there  was  a  tilting 
of  the  floor  of  the  room  in  which  the  balance  was  placed, 
which  had  to  be  allowed  for  in  the 
discussion  of  the  results.  One  of 
the  details  of  Prof.  Poynting's  in- 
vestigation illustrates  a  refinement 
of  modern  science  due  to  Lord 
Kelvin.  Since  the  attracting  force 
is  so  small,  it  is  evident  that  the 
movements  of  the  long  pointer  of 
the  ordinary  balance  which  indi- 
cates the  difference  in  weight  in 
the  balance  pans  would  be  too 
small  to  observe.  To  make  these 
small  movements  perceptible  and 
FlG  j  also  measurable,  a  small  arm  or 

bracket,  B,  was  fixed  to  the  pointer 
of  the  balance.  One  end  of  a  spider  thread  or  quartz 
fibre  by  which  the  mirror  was  suspended  was  attached 
to  this  bracket,  and  the  other  end  to  a  fixed  support, 
S,  independent  of  the  balance.  The  angle  through 
which  the  mirror,  M,  turns  for  a  given  motion  of  the 


MEASUREMENTS  IN  ELECTRICITY.  15 

pointer  is  inversely  as  the  distance  between  it  and  the 
fixed  point,  "  so  that  by  diminishing  this  distance  the 
sensibility  of  the  arrangement  may  be  almost  indefi- 
nitely increased."  In  Prof.  Poynting's  experiments, 
taking  4  millimetres  as  the  distance  between  the  threads 
and  supposing  the  bracket  to  be  600  millimetres  below 
the  knife  edge  of  the  balance,  the  mirror  turns  through 
an  angle  150  times  as  great  as  that  through  which 
the  beam  turns.  The  observation  of  the  angular  move- 
ment of  the  mirror  was  observed  by  a  telescope  placed 
at  a  distance.  The  movement  of  the  mirror,  it  is  evi- 
dent, sweeps  a  beam  of  light  through  space.  A  move- 
ment of  6  6  ^  0  7  of  an  inch  could  thus  be  detected  in 
the  motion  of  the  attracting  masses.  An  idea  of  the 
amount  of  the  attraction  between  small  bodies  can  also  be 
gained  from  a  recent  investigation  of  Prof.  C.  Y.  Boys, 
who  finds  that  the  force  with  which  two  spheres  weigh- 
ing a  gramme  each  (about  -^  of  a  pound)  with  their 
centres  1  centimetre  (about  -^  of  an  inch)  apart  attract 
one  another  is  nearly  YOOOOOOO^  °^  a  dyne,*  and  that  the 
mean  density  of  the  earth  is  5*5270  times  that  of  water. 
Although  the  force  of  attraction  between,  bodies  of 
small  magnitude  requires  for  its  detection  apparatus  of 
extreme  delicacy,  yet  when  one  of  the  attracting 
bodies  is  large,  like  the  earth,  the  force  of  attraction 
between  it  and  even  minute  bodies  becomes  appreciable 
to  our  senses.  Take,  for  instance,  the  effect  of  gravita- 
tion in  regulating,  so  to  speak,  the  transformations  of 
energy  in  our  atmosphere.  "When  water  is  heated,  the 
warm  water,  being  less  dense  and  therefore  having  less 

*  A  dyne  is  the  force  which,  acting  upon  a  gramme  for  a  second, 
generates  a  velocity  of  a  centimetre  per  second.  The  force  of 
gravity  acting  upon  a  gramme  generates  a  velocity  of  981  centi- 
metres per  second. 


16  WHAT  IS  ELECTRICITY? 

its  energy,  or,  in  other  ™rds,  it  must  be  cooled 


total  of  water  which  descends  as    rain  or  snow  i 
the  United  States.  . 

«  To  get  some  conception  of  this  enormous  mass  o. 
water  we  may  compare  it  with  the  contents  of  the 
Great  Lakes,  and  an  approximate  comparison  is  near 
enough     Lake  Ontario  is  about  200  miles  long  and  <0 
broad  and  its  average  depth  is  about  40  fathoms.     It 
therefore  contains  about   636  cubic  miles  of  water. 
The  annual  rainfall  would  fill  it  two  times  and  leave 
something  over  for  a  third  time.     Lake  Michigan  is 
about  310  by  70  miles  and  has  an  average  depth  of 
about  50  fathoms,  and  consequently  contains  about 
1,233  cubic  miles  of  water.     The  average  annual  rain- 
fall would  fill   Lake  Michigan  and  leave  174  cubic 
miles  over.    Four  years  of  rainfall  would  probably  be 
enough  to  fill  all  the  Great  Lakes. 

"  The  amount  of  mechanical  work  which  the  raising 
of  tliis  involves  is  enormous,  and  the  ordinary  concep- 


Bulletin  C,  1894. 


MEASUREMENTS  IN  ELECTRICITY.  1? 

tion  of  it  is  quite  inadequate.  Some  idea  of  it  can  be 
reached  as  follows :  One  inch  of  rain  per  acre  makes 
22,624  gallons,  which  equals  226,613  pounds.  On  a 
square  mile  the  inch  of  water  would  weigh  72,516*4: 
tons  (of  2,000  pounds  each).  A  cubic  mile  of  water 
would  be  this  weight  X  5,280  X  12  =  4,593,639,104  tons, 
or,  if  the  average  temperature  is  a  little  above  39°  F., 
=  4,500,000,000  tons.  The  total  weight  of  our  rainfall 
(excluding  Alaska)  would  be  this  multiplied  by  14'OT. 
This  gives  the  enormous  quantity  of  6,332,000,000,000 
tons.  Let  us  take  as  a  unit  of  handy  measurement  the 
weight  of  one  of  the  lakes,  say  Ontario — 636  cubic 
miles.  A  cubic  mile  of  water  weighs  4,500,000,000 
tons.  Hence,  Ontario  weighs  2,862,000,000,000  tons. 
Our  average  rainfall,  weighing  6,332,000,000,000  tons, 
is  therefore  2*2  Ontarios.  The  rain  descends  from 
clouds  which  average  half  a  mile  in  height,  and  in  rais- 
ing the  water  to  this  height  before  falling  Kature  must 
perform  the  work  of  lifting  3,166,000,000,000  tons  one 
mile  per  year,  or  Tl  Ontarios.  This,  in  work  per  day, 
is  nearly  9,000,000,000  tons  lifted  one  mile,  and  re- 
duces to  something  like  a  lift  of  100,000  tons  per 
second.  A  ton  lifted  one  mile  per  second  is  19,200 
horse  power.  The  work  done  by  Mature,  therefore,  in 
raising  the  rainfall  to  the  clouds  is  equivalent  to  100,- 
000  X  19,200  horse  power,  or  1,920,000,000  continu- 
ous horse  power,  or  the  work  of  5,000,000,000  horses 
working  ten  hours  a  day — perhaps  a  thousand  times  as 
many  horses  as  there  are  in  the  United  States. 

The  effect  of  gravitation  in  protecting  the  flora  of 
our  forests  from  evaporation  is  also  very  marked,  for 
it  tends  to  counteract  the  upward  springing  of  the 
branches  toward  the  light  and  to  bend  them  over  the 
earth.  On  certain  portions  of  the  sandy  lands  of  Cape 


18  WHAT  IS  ELECTRICITY? 

Cod  it  is  said  that  wooded  tracts  have  been  destroyed 
by  cutting  off  the  low-spreading  branches  of  the  trees 
and  clearing  up  the  space  beneath  the  trees.  Gravita- 
tion had  done  its  part  toward  preventing  evaporation. 
A  friend,  in  commenting  upon  the  very  erect  attitude 
of  one  who  was  proud  of  a  scientific  achievement,  re- 
marked that  in  time  the  force  of  gravitation  would  cor- 
rect all  that. 

The  force  of  gravitation  gives  us  our  units  for 
measuring  the  manifestations  of  electricity.  Does  it 
also  aid  us  in  comprehending  the  forces  of  attraction 
which  we  perceive  to  be  acting  between  magnets,  and 
electrified  pith  balls? — forces  which  are  analogous  in 
their  mathematical  expression  to  the  force  of  attraction 
between  two  masses,  or,  in  other  words,  to  the  force  of 
gravitation. 

Certain  students  of  the  motion  of  fluids  endeavor  to 
explain  gravitation  by  the  movement  of  the  ether.  They 
suppose  that  the  ether  is  moving  throughout  space — a 
great  ether  ocean  with  a  definite  tide.  According  to 
this  theory,  the  ether  passes  through  the  sun  and  the 
planets  with  more  or  less  difficulty,  and  its  motion  forces 
the  particles  of  matter  together. 

The  theory  that  gravitation  can  be  explained  by  the 
motions  of  the  ether  is  interesting  from  the  point  of 
view  that  it  apparently  unifies  our  conceptions  of  trans- 
formations of  energy  ;  for  we  shall  see  that  the  ether  is 
supposed  with  much  reason  to  be  the  medium  by  means 
of  which  the  waves  of  light,  heat,  and  electricity  are 
conveyed  to  us  from  the  sun.  If  we  could  also  show 
that  the  force  of  gravitation  results  from  the  different 
rates  of  flow  of  this  medium  through  the  particles  or 
around  the  atoms  of  bodies,  we  might  bring  gravitation 
into  closer  connection  with  electro-magnetism.  The  at- 


MEASUREMENTS  IN  ELECTRICITY.  19 

tempt  to  show  that  the  differential  flow  of  a  medium 
like  the  ether  through  empty  space  and  through  space 
filled  with  material  particles  is  one  of  great  difficulty 
from  both  the  experimental  and  mathematical  point  of 
view.  We  have  never  detected  any  effect  of  the  ether 
upon  the  motion  of  bodies,  and  experiments  upon  the 
attraction  of  masses  suspended  in  a  moving  fluid  like 
water  are  inconclusive,  since  water  is  very  different 
from  the  fluid  we  call  ether.  If  we  suspend,  for  in- 
stance, in  a  trough  of  moving  water  two  cylinders  of 
gauze  of  different  mesh,  these  cylinders  will  be  appa- 
rently attracted  to  each  other  by  degrees  varying  with 
the  difference  in  the  fineness  of  the  meshes  of  the 
gauze.  "We  must  reflect,  however,  that  we  can  not 
reason  from  the  motions  of  a  fluid  possessing  viscosity, 
like  water,  to  the  motions  of  an  ether  which  is  not  vis- 
cous. 

If  we  suppose  that  the  attraction  of  gravitation 
arises  from  a  stress  in  the  ether,  this  stress  would  have 
to  be  enormous. 

Williamson  *  calculates  that  the  amount  of  the  ether 
stress  at  the  earth's  surface  to  account  for  gravitation 
would  be  4,000  tons  on  the  square  inch. 

"  Lord  Kelvin  has  shown  that  if  we  suppose  all 
space  to  be  filled  with  a  uniform  incompressible  fluid, 
and  if  we  suppose  either  that  material  bodies  are 
always  generating  and  emitting  this  fluid  at  a  constant 
rate,  the  fluid  flowing  off  to  infinity,  or  that  material 
bodies  are  always  absorbing  and  annihilating  the  fluid, 
the  deficiency  flowing  in  from  infinite  space,  then  in 
either  of  these  cases  there  would  be  an  attraction  be- 


*  Introduction  to  the  mathematical  theory  of  the  stress  and 
strain  of  elastic  solids. 


. 


2Q  WHAT  IS  ELECTRICITY! 

tween  any  two  bodies  inversely  as  the  square  of  the 
distance.  If,  however,  one  of  the  bodies  were  a  gen- 
erator of  the  fluid  and  the  other  an  absorber  of  it,  the 
bodies  would  repel  each  other."  Maxwell  *  in  criticis- 
ing this  supposition,  remarks  that  it  seems  to  require 
anabsorption  or  annihilation  of  matter. 

An  attempt  to  explain  the  force  of  gravitation  by 
the  impact  of  corpuscles  was  brought  forward  by  Le 
Sage,  who  supposed  that  all  bodies  are  bombarded  by 
an^immense  number  of  corpuscles  which  are  flying 
about  with  great  velocity.    One  mass,  therefore,  partial- 
ly shades  or  protects  another  neighbouring  mass  from 
the  impacts  of  such  corpuscles,  and  the  masses  being 
more  bombarded  on  the  sides  that  are  not  opposed  are 
apparently  attracted  toward  each  other.     This  theory 
gives  a  very  plausible  explanation  of  the  mysterious  force 
of  gravitation;  but  Clerk  Maxwell,  applying  the  doc- 
trine of  the  conservation  and  transformation  of  energy, 
proved  that  the  theory  must  be  supplemented  by  other 
theories  which  are  as  little  supported  as  the  theory  itself. 
Maxwell  shows  that  the  velocity  of  Le  Sage's  cor- 
puscles must  be  enormously  greater  than  that  of  any  of 
the  heavenly  bodies,  otherwise  they  would  act  as  a  re- 
sisting medium  and  oppose  the  motion  of  the  plants. 
The  energy  of  the  corpuscles  would  be  enormous.     The 
rate  at  which  it  must  be  spent  in  order  to  maintain  the 
gravitating  property  of  a  single  pound    is    at   least 
millions  of  millions  of  foot  pounds  per  second.     If  any 
appreciable  amount  of  this  energy  is  communicated  to  a 
body  in  the  form  of  heat,  the  amount  of  heat  so  gen- 
erated would  raise  the  whole  material  universe  in  a  few 
seconds  to  a  white  heat. 


*  Encyclopaedia  Britannica,  Gravitation. 


MEASUREMENTS  IN  ELECTRICITY.  2L 

"Prof.  Challis  has  investigated  the  mathematical 
theory  of  the  effect  of  waves  of  condensation  and  rare- 
faction "  in  an  elastic  fluid  on  bodies  immersed  in  the 
fluid.  He  concludes  that  the  effect  of  such  waves  would 
be  to  attract  the  body  toward  the  centre  of  agitation  or 
to  repel  it  from  that  centre,  according  as  the  wave's 
length  is  very  large  or  very  small  compared  with  the 
dimensions  of  the  body. 

A  tuning  fork  set  in  vibration  attracts  a  delicately 
suspended  body.  Lord  Kelvin  shows  that  in  fluid  mo- 
tion the  average  pressure  is  least  when  the  average 
energy  of  motion  is  greatest.  The  wave  motion  is 
greatest  near  the  fork  and  the  pressure  there  is  least, 
and  the  suspended  body  is  therefore  urged  toward  the 
fork.  Maxwell  remarks  that  all  the  theories  which 
have  been  brought  forward  to  explain  gravitation — 
namely,  Le  Sage's  corpuscle  theory,  the  generation  or 
absorption  of  fluid  by  bodies  under  pressure,  the  wave 
theory — require  the  expenditure  of  work.  "  According 
to  such  hypotheses  we  must  regard  the  processes  of 
Mature  not  as  illustrations  of  the  great  principle  of  the 
conservation  of  energy,  but  as  instances  in  which,  by  a 
nice  adjustment  of  powerful  agencies  not  subject  to  thisf 
principle,  an  apparent  conservation  of  energy  is  main- 
tained. Hence  we  are  forced  to  conclude  that  the  ex- 
planation of  the  cause  of  gravitation  is  not  to  be  found 
in  these  three  hypotheses." 

It  is  interesting  to  follow  the  working  of  Faraday's 
mind  on  the  subject  of  gravitation.  Having  opened  a 
great  field  in  the  transformations  of  electricity  into 
magnetism,  and  of  magnetism  into  electricity,  his  mind 
sought  to  embrace  the  force  of  gravitation  in  a  generali- 
zation which  should  include  it  with  those  of  electricity. 
An  experiment  by  Faraday  is  always  worthy  of  respect- 


22  WHAT  IS  ELECTRICITY! 

fid  consideration ;  but  in  following  him  in  this  excursion 
we  feel  in  a  certain  sense  like  one  who  accompanies  a 
voyager  on  a  strange  sea.     There  was  nothing  to  guide 
him      No  experiments,  no  shadowy  intimations  of  rela- 
tionship, such  as  had  always  accompanied  the  manifesta- 
tions of  electricity  and  magnetism.    His  mind  naturally 
rested  upon  the  electrical  methods  which  had  proved  so 
fruitful  in  discovering  the  laws  of  induction,  and  he 
arranged  his  apparatus  as  follows.    Since  he  knew  noth- 
ing about  gravitation  except  its  measure  and  the  direc- 
tion in  which  it  acted,  he  determined  to  apply  the  anal- 
ogies of  the  laws  of  the  transformation  of  energy,  to  see 
if  some  electrical  work  was  not  done  when  a  ball  of 
glass  or  wood  was  "allowed  to  fall  along  the  lines  of 
gravitation  and  was  immediately  drawn  up  against  the 
force  of  gravitation.    It  is  evident  that  his  fruitful  con- 
ception of  space  filled  with  lines  of  magnetic  and  elec- 
tric force  occurred  to  him.     A  coil  of  wire  moved  in  a 
certain  manner  across  magnetic  lines  of  force  showed 
electrical  disturbances.     Such  a  coil,  however,  did  not 
give  any  indications,  no  matter  how  it  was  moved  in  ref- 
erence to  the  lines  of  force,  of  gravitation.    The  needle 
,of  a  delicate  galvanometer  remained  absolutely  quiet  if 
the  galvanometer  were  connected  with  a  coil  which  was 
moved  across  or  along  the  lines  of  gravitating  force. 
"What  led  Faraday  to  suppose  that  an  effect  could  be 
observed  by  allowing  a  nonmagnetic  body  like  glass  to 
fall  through  the  coil  and  then  to  be  drawn  up  through  the 
coil  against  gravitation  ?     In  one  case  work  was  being 
done  by  the  falling  body,  and  in  the  other  case  work 
was  done  against  the  pull  of  gravitation.     It  is  evident 
that  Faraday  expected  the  electrical  equilibrium  of  the 
coil  to  be  disturbed  by  this  transformation  in  the  space 
outside  it. 


MEASUREMENTS  IN  ELECTRICITY.  23 

Faraday,  in  speaking  of  his  efforts,  remarks : 

"  The  long  and  constant  persuasion  that  all  forces  of 
Nature  are  mutually  dependent,  having  one  common 
origin,  or  rather  being  different  manifestations  of  one 
fundamental  power,  has  made  me  often  think  upon  the 
possibility  of  establishing  by  experiment  a  connection 
between  gravity  and  electricity,  and  so  by  introducing 
the  former  into  the  group,  the  chain  of  which,  including 
also  magnetism,  chemical  force,  and  heat,  to  bind  so 
many  and  such  varied  exhibitions  of  force  together  by 
common  relations." 

"  Hypothesis :  Two  bodies  moved  toward  each  other 
by  the  force  of  gravity  currents  of  electricity  might  be 
developed  either  in  them  or  in  the  surrounding  matter 
in  one  direction ;  and  that,  as  they  were  by  extra  force 
moved  from  each  other  against  the  power  of  gravita- 
tion, the  opposite  currents  might  be  produced.  A  body 
was  allowed  to  fall  with  a  helix,  and  afterward  through 
a  helix.  Various  bodies  were  used,  notably  copper,  bis- 
muth, glass,  sulphur,  gutta-percha.  It  was  thought 
that  the  stopping  of  the  up-and-down  motion  in  the 
line  of  gravity  would  produce  contrary  effects  to  the 
coming  on  of  the  motion,  and  that  whether  the  stopping 
was  sudden  or  gradual ;  also  that  a  motion  downward 
quicker  than  that  which  gravity  could  communicate 
would  give  more  effect  than  the  gravity  result  by  itself, 
and  that  a  corresponding  increase  in  the  velocity  up- 
ward would  be  proportionally  effectual.  A  machine 
was  devised  which  could  give  a  rapidly  alternating  up- 
and-down  motion. 

"  Here  end  my  trials  for  the  present.  The  results  are 
negative.  They  do  not  shake  my  strong  feelings  of  the 
existence  of  a  relation  between  gravity  and  electricity, 
though  they  give  no  proof  that  such  a  relation  exists." 


24  WHAT  IS  ELECTRICITY! 

We  shall  now  enter  upon  the  study  of  the  mysteries 
of  electricity  by  means  of  the  more  mysterious  force  of 
gravitation ;  for,  as  I  hav&  said,  our  measurements  of 
force  in  general  are  based  upon  measurements  of  the 
force  of  gravitation.  ,  Our  great  unit  in  the  study  of 
transformations  of  energy  is  the  mechanical  equivalent 
of  heat,  and,  as  we  have  already  pointed  out,  this  is  ex- 
pressed in  terms  of  a  mass  lifted  a  certain  height  against 
the  force  of  gravitation. 


CHAPTER  III. 

MAGNETISM. 

To  an  American,  the  question  "What  is  electricity  ? 
has  a  great  national  interest,  for  Count  Rumford, 
Benjamin  Franklin,  and  Joseph  Henry  have  been  like 
electric  lights  spaced  on  a  dark  path  of  a  high  moun- 
tain, and  we  are  slowly  ascending  to  a  summit,  having 
been  guided  by  the  rays  of  their  genius.  In  America, 
too,  the  practical  applications  of  electricity  have  ex- 
tended with  such  swiftness  "that  one  might  say  that 
there  is  something  in  the  manifold  transformations  of 
electricity  peculiarly  congenial  to  the  American  tem- 
perament. While  the  reader,  however,  may  be  willing 
to  admit  the  claims  of  Benjamin  Franklin  and  of  Joseph 
Henry  to  be  pioneers  in  the  subject  of  electricity,  he 
may  doubt  the  justice  of  including  Count  Rumford 
with  these  workers  on  the  question,  What  is  electricity  ? 

Our  final  answer  to  this  great  question  will  neces- 
sarily embrace  the  labours  of  Count  Rumford  in  the 
subject  of  heat ;  for  his  celebrated  experiment  on  the 
heat  developed  in  the  boring  of  a  cannon  in  Munich 
drew  mens'  attention  to  the  transformation  of  mechan- 
ical work  into  heat,  and  led  them  to  reflect  on  the  con- 
servation of  energy.  Guided  by  this  great  theory,  we 
have  learned  that  light,  heat,  electricity,  and  magnetism 
can  be  studied  and  embraced  under  one  head — that  of 


26  WHAT  IS  ELECTRICITY! 

the  transformations  of  energy.  Electricity  no  longer 
stands  apart,  a  mysterious  force,  as  Franklin  regarded  it, 
having  no  connection  with  light  or  heat.  It  is  now 
seen  that  we  can  not  study  it  apart  from  the  manifesta- 
tions of  the  latter. 

Knowing  that  Count  Kumford,  when  a  boy,  walked 
from  Woburn  to  Cambridge,  a  distance  of  eight  miles, 
to  attend  the  lectures  of  Prof.  John  Winthrop,  the  first 
Professor  of  Physics  at  Harvard  University,  I  was  in- 
terested to  ascertain  how  much  he  learned  of  the  subject 
of  electricity.  In  the  college  archives  I  found  a  time- 
worn  notebook  in  the  handwriting  of  Winthrop,  and 
among  the  notes  of  excellent  lectures  on  astronomy  and 
a  few  on  light  and  heat  I  found,  apparently,  that  but 
one  lecture  had  been  given  on  magnetism  and  one  on 
electricity.  As  a  curious  illustration  of  the  extent  of 
our  knowledge  of  a  great  subject  less  than  a  century 
and  a  half  ago,  I  give  the  main  portion  of  his  notes 
on  the  lecture  on  electricity  in  1750  :  "  If  a  flaxen 
string  be  extended  and  supported,  and  at  one  end  an 
excited  tube  be  applied,  light  bodies  will  be  attracted, 
and  that  at  the  distance  of  1,200  feet  at  the  other  end. 
This  electricity  since  the  year  1743  has  made  a  consid- 
erable noise  in  the  world,  upon  which  it  is  supposed 
several  of  the  (at  present)  hidden  phenomena  of  Mature 
depend.  .  .  .  Men  have  been  so  electrized  as  to  have 
considerable  light  round  their  heads  and  bodies,  not  un- 
like the  light  represented  around  the  heads  of  saints  by 
the  painters." 

The  entire  apparatus  to  illustrate  the  subject  of 
electricity  and  magnetism  in  Harvard  University  until 
the  year  1820  consisted  merely  of  two  Franklin  elec- 
trical machines,  a  collection  of  Leyden  jars,  and  small 
apparatus  to  illustrate  the  effects  of  electrical  attractions 


MiGNETISM. 


27 


and  repulsions  shown  by  electrified  pith  balls  or  similar 
light  objects.  I  have  had  the  Franklin  machine  pho- 
tographed beside  a  modern  electrical  machine  which 
can  be  carried  around  in  the  arms,  and  which  has 
many  times  the  efficiency  of  the  machine  employed  by 
Franklin.  Fig.  2  shows  a  cut  from  this  photograph. 
During  the  days,  therefore,  of  Prof.  Winthrop  the 
knowledge  of  electrical  phenomena  was  extremely 
small.  It  was  confined  to  the  observation  of  the  attrac- 


tion  of  magnets  and  of  the  phenomena  of  frictional 
electricity.  America,  however,  in  the  year  1750  knew 
as  much  as  Europe,  and  the  physical  cabinet  of  Harvard 
University  was  not  more  poverty-stricken  than  that  of 
the  University  of  Leyden.  Our  advance  since  1750 
has  been  due  to  the  accurate  quantitative  investigation 
of  the  transformations  of  energy ;  and  although  Ben- 
jamin Franklin's  brilliant  experiment  in  establishing 
the  identity  between  the  manifestations  of  lightning 


28  WHAT  IS  ELECTRICITY! 

and  those  of  the  ordinary  electrical  machine  is  often  re- 
ferred to  as  the  beginning  of  our  real  knowledge  of 
electricity,  I  should  say  that  the  experiment  of  Count 
Kumf ord  in  boring  the  cannon  has  had  far  more  real 
influence  in  the  development  of  true  ideas  in  regard  to 
the  transformations  of  energy  in  which  electricity  plays 
such  an  important  part. 

In  hitherto  unpublished  letters  of  Count  Kumford 
to  Pictet,  of  Geneva,  in  1797,  now  in  the  possession  of 
the  American  Academy  of  Arts  and  Sciences,  Boston, 
he  shows  how  clearly  he  had  seized  upon  a  fundamental 
idea  of  the  transformations  of  energy : 

"Your  friend  Mr.  Joly  will  perhaps  have  men- 
tioned to  you  a  late  experiment  of  mine  in  which  I 
caused  more  than  four  gallons  of  water  to  boil  without 
fire — merely  by  the  heat  generated  by  the  friction  of  two 
metals  rubbed  against  each  other.  I  have  just  finished  a 
calculation  by  which  it  appears  that  the  heat  generated 
equably,  or  the  stream  of  heat  which  flowed  with  a  uni- 
form velocity — if  I  may  so  express  myself — was,  in  one 
of  my  experiments,  equal  to  that  generated  equably  in 
the  combustion  of  nine  middling-sized  wax  candles  all 
burning  together  or  at  the  same  time. 

"Ajs  the  machinery  which  produced  the  friction 
which  generated  this  heat  could  easily  be  put  and  kept 
in  motion  by  the  strength  of  one  strong  horse,  we  see 
how  much  heat  could  be  generated  by  the  strength  of 
animals,  without  either  fuel,  light,  or  chemical  decom- 
position. 

"  I  am  just  now  engaged  in  writing  a  paper  on  the 
subject  which  I  mean  to  send  to  the  Eoyal  Society. 
The  results  of  my  experiments  seem  to  me  to  prove  to 
a  demonstration  that  there  is  no  such  thing  as  an  igneous 
fluid,  and  consequently  that  caloric  has  no  real  exist- 


MAGNETISM.  29 

ence.  You  must  not,  however,  raise  your  expectations 
too  high  respecting  my  experiments.  Though  they  were 
made  on  a  large  scale  and  conducted  with  care,  there 
was  nothing  very  new  or  very  remarkable  about  them ; 
and  as  to  their  results,  they  prove  only  this  single  fact 
(of  which  most  probably  you  never  had  any  doubt),  that 
the  heat  generated  by  friction  is  inexhaustible,  even 
when  the  bodies  rubbed  together  are  to  all  appearance 
perfectly  insulated  or  put  into  a  situation  in  which  it  is 
evidently  impossible  for  them  to  receive  from  the  other 
bodies  the  heat  they  are  continually  giving  off. 

"It  appears  to  me  that  that  which  any  insulated 
body  or  system  of  bodies  can  continue  to  give  off  with- 
out limitation  can  not  be  a  material  substance.  A  bell 
when  struck  with  a  hammer  gives  off  sound,  but  I  do 
not  think  it  would  be  speaking  philosophically  to  call 
sound  a  material  substance." 

In  another  letter  to  Prof.  Pictet,  dated  Paris,  May  4, 
1804,  he  says,  "  I  am  persuaded  that  I  shall  live  a  suffi- 
ciently long  time  to  have  the  satisfaction  of  seeing  caloric 
interred  with  phlogiston  in  the  same  tomb." 

The  science  of  electricity  took  an  immense  stride  as 
soon  as  the  transf ormations  of  energy  were  studied  from 
a  mechanical  standpoint — in  other  words,  from  the  stand- 
point of  Count  Eumford — and  as  soon  as  men  aban- 
doned theories  of  subtle  fluids  and  began  to  measure  the 
forces  of  attraction  and  repulsion  and  the  equivalence 
between  motion  and  the  energy  it  makes  manifest, 
whether  we  convert  this  energy  into  heat  or  into  elec- 
tricity. Count  Eumford  saw  clearly  only  the  transfor- 
mation of  mechanical  work  into  heat,  and  the  relation 
between  the  work  of  a  horse  in  producing  this  trans- 
formation and  the  food  which  he  eats.  Indeed,  he 
made  a  rough  calculation  of  this  transformation.  We 


30  WHAT  IS  ELECTRICITY? 

shall  see  that  as  soon  as  Faraday  showed  that  motion 
could  also  be  converted  into  electricity,  and  when  Joule 
showed  the  equivalence  between  the  energy  of  move- 
ment and  the  electrical  energy  produced,  we  entered 
upon  the  new  era  of  electricity — an  era  which  is  charac- 
terized by  our  study  of  the  transf ormations  of  energy. 

In  the  subject  of  electricity  delicate  measuring  in- 
struments have  played  a  most  important  part.  In  gen- 
eral these  instruments  measure  attractions  and  repul- 
sions. Before  the  year  1800  there  were  no  delicate 
instruments  for  measuring  such  forces.  A  compass  on 
a  pivot  was  the  most  sensitive  instrument  that  was  used 
to  study  magnetism,  and  the  electrified  pith  balls  or 
the  suspended  gold  leaves  constituted  the  measuring 
apparatus  in  all  that  was  known  then  of  electricity. 
The  subject  of  electricity  took  its  great  stride  not  from 
the  use  of  the  instruments  employed  by  Franklin  or 
from  the  side  of  the  subject  investigated  by  him,  but 
rather  from  the  side  of  magnetism.  It  was  the  move- 
ments of  a  magnetized  needle  that  led  Faraday  to  his 
great  discovery  of  induction  and  the  conversion  of  mo- 
tion into  electricity.  "We  have  learned  since  1830  a 
great  deal  about  the  magnetic  properties  of  soft  iron 
under  electrical  influences,  we  know,  however,  little  more 
of  the  properties  of  the  loadstone  and  of  permanent  mag- 
nets, and  of  the  magnetism  of  the  earth,  than  was  known 
to  Count  Kumf  ord  or  to  Sir  Isaac  Newton.  Both  of  these 
philosophers,  I  imagine,  thought  that  if  all  the  loadstones 
in  the  world,  the  earth's  magnetism,  and  the  permanent 
magnets,  were  destroyed  that  magnetism  would  disap- 
pear from  the  sum  of  the  forces  whose  origin  and  mani- 
festations perplex  the  human  mind.  They  had  no 
conception  of  the  possibility  of  producing  a  magnetic 
condition  in  iron  by  means  of  a  wire  wrapped  around  the 


MAGNETISM.  31 

iron  and  connected  with  a  voltaic  cell ;  for  the  principles 
of  electro-magnetism  were  not  discovered  until  1819. 
To-day,  if  there  should  be  such  a  destruction  as  we  have 
outlined — namely,  that  of  loadstones,  permanent  mag- 
nets, and  the  disappearance  of  the  earth's  magnetic 
force — the  world  would  be  apparently  only  incommoded 
by  the  substitution  of  observations  on  the  sun  and  stars 
for  the  observation  of  the  ship's  compass.  We  could 
produce  permanent  magnets  and  powerful  electro-mag- 
nets at  pleasure  and  in  any  quantity  by  the  use  of  vol- 
taic cells  or  the  dynamo. 

The  question,  however,  What  is  magnetism?  is 
closely  allied  to  the  question,  What  is  electricity  ?  and 
before  entering  upon  the  phenomena  of  electro-mag- 
netism it  is  well  to  consider  the  force  of  magnetism. 

In  a  letter  to  M.  Dubourg,  dated  London,  March 
10,  1773,  Franklin  thus  explains  his  views  of  mag- 
netism : 

"  SIB  :  As  to  the  magnetism  which  seems  produced 
by  electricity,  my  real  opinion  is  that  these  two  powers 
of  Nature  have  no  affinity  with  each  other,  and  that  the 
apparent  production  of  magnetism  is  purely  accidental. 
The  matter  may  be  explained  thus  : 

"  1.  The  earth  is  a  great  magnet. 

"  2.  There  is  a  subtle  fluid,  called  the  magnetic  fluid, 
which  exists  in  ah1  ferruginous  bodies,  equally  attracted 
by  all  their  parts,  and  equally  diffused  through  their 
whole  substance ;  at  least  where  the  equilibrium  is  not 
disturbed  by  a  power  superior  to  the  attraction  of  the 
iron. 

"  3.  This  natural  quantity  of  the  magnetic  fluid  which 
is  contained  in  a  given  piece  of  iron  may  be  put  in  mo- 
tion so  as  to  be  more  rarefied  in  one  part  and  more  con- 
densed in  another ;  but  it  caii  not  be  withdrawn  by  any 


32  WHAT  IS  ELECTRICITY? 

force  that  we  are  yet  made  acquainted  with  so  as  to 
leave  the  whole  in  a  negative  state,  at  least  relatively 
to  its  natural  quantity ;  neither  can  it  be  introduced  so 
as  to  put  the  iron  into  a  positive  state  or  render  it  plus. 
In  this  respect,  therefore,  magnetism  differs  from  elec- 
tricity. 

"  4.  A  piece  of  soft  iron  allows  the  magnetic  fluid 
which  it  contains  to  he  put  in  motion  by  a  moderate 
force,  so  that,  being  placed  in  a  line  with  the  magnetic 
pole  of  the  earth,  it  immediately  acquires  the  property 
of  a  magnet,  its  magnetic  fluid  being  drawn  or  forced 
from  one  extremity  to  the  other ;  and  this  effect  con- 
tinues as  long  as  it  remains  in  the  same  position,  one  of 
its  extremities  becoming  positively  magnetized  and  the 
other  negatively.  This  temporary  magnetism  ceases  as 
soon  as  the  iron  is  turned  east  and  west,  the  fluid  imme- 
diately diffusing  itself  equally  through  the  whole  iron, 
as  in  its  natural  state." 

In  the  same  letter  he  still  further  enforces  his  fluid 
theory  of  magnetism. 

The  molecular  theory  of  magnetism,  which  may  be 
said  to  appeal  strongly  to  chemists,  since  it  seems  more 
or  less  in  consonance  with  the  theory  of  attraction  of 
molecules,  has  been  advanced  by  various  physicists,  par- 
ticularly by  Weber.  This  theory  supposes  that  the 
molecules  of  a  bar  of  iron  are  small  magnets  which,  when 
the  bar  is  unmagnetized,  point  indiscriminately  in  all 
directions,  but  when  it  is  magnetized  a  certain  number 
of  these  little  magnets  point  in  a  definite  direction. 
Many  of  the  phenomena  of  magnetism  give  great  colour 
to  this  theory,  and  it  may  be  said  to  be  satisfactory  until 
we  examine  the  action  of  magnetism  on  light ;  then  we 
perceive  that  there  are  not  only  attractive  forces  between 
molecules  of  the  iron,  but  also  rotary  motions  in  the 


MAGNETISM.  33 

medium  within  and  around  the  magnet,  and  in  order  to 
comprehend  the  phenomenon  of  magnetism  we  are  com- 
pelled to  assume  a  medium  between  the  molecules  and 
to  attribute  rotary  or  vortex  movements  to  it.  "We  shall 
return  to  this  more  comprehensive  theory  later.  The 
truth  seems  to  lie  between  the  two  theories.  It  is  cer- 
tain that  the  molecular  condition  of  iron  and  steel  great- 
ly influences  its  magnetic  condition.  For  instance,  if  a 
permanent  steel  magnet  is  struck  with  a  hammer  it  loses 
a  part  of  its  magnetism.  The  molecules  appear  to  be 
shaken  out  of  the  position  which  they  had  taken  on  be- 
ing magnetized.  If  a  bar  of  soft  iron  is  held  in  the  line 
of  magnetic  dip — that  is,  the  line  indicated  by  a  long 
magnetic  needle  suspended  from  its  middle,  the  line 
along  which  a  south  pole,  if  it  could  be  separated  from 
its  north  pole,  would  travel  toward  the  north  pole  of 
the  earth — and  is  struck  a  sharp  blow  with  a  hammer,  it 
will  become  magnetic.  Its  condition  can  be  tested  by 
presenting  its  ends  to  an  ordinary  compass.  If,  on  the 
other  hand,  it  is  held  east  and  west  and  struck,  it  can  be 
deprived  of  its  magnetic  state.  It  is  found  that  iron 
ships  become  magnetic  in  certain  portions  under  the 
hammering  and  vibrations  to  which  they  are  subjected 
in  shipyards.  If  the  molecules  of  steel  are  subjected  to 
high  temperature — for  instance,  to  a  white  heat — the 
steel  is  deprived  of  its  magnetism.  Yery  interesting  work 
in  this  field  has  been  done  by  Profs.  Hopkinson  and 
Ewing.  Some  years  since  I  examined  the  behaviour  of 
a  piece  of  steel  when  cooled  to  80°  C.  below  zero.  I 
found  that  its  magnetic  condition,  measured  by  what  is 
called  its  magnetic  moment,  was  diminished  50  per 
cent.  In  calling  attention  to  this  observation  I  remarked 
that  every  piece  of  steel  had  its  idiosyncrasies,  and  I 
hoped  to  continue  my  observations  with  other  speci- 


34  WHAT  IS  ELECTRICITY! 

mens.  Prof.  Dewar,  who  has  made  some  remarkable 
observations  on  molecular  conditions  at  very  low  tem- 
peratures, has  lately  examined  the  effect  of  great  cold 
on  magnetism,  and  I  can  not  do  better  than  to  quote  his 
words  hi  a  paper  on  the  scientific  uses  of  liquid  air : 

"  Prof.  Trowbridge  examined  the  effect  of  a  tem- 
perature of  —80°  C.  upon  a  permanent  magnet,  and 
came  to  the  conclusion  that  the  magnetic  moment 
was  diminished  by  about  50  per  cent.  Prof.  Ewing 
found  that  an  increase  of  temperature  of  150°  C.  above 
10°  caused  a  reduction  of  the  magnetic  moments  of  a 
bar  magnet  by  about  40  per  cent,  and  that  the  magnet 
on  cooling  recovered  its  original  state.  This  result 
would  lead  us  to  expect  that  if  the  same  law  is  followed 
below  the  melting  point  of  ice  as  Ewing  found  above  it, 
then  a  bar  magnet  cooled  to  —182°  C.  ought  to  gain  in 
magnetic  moment  something  like  30  to  50  per  cent. 
The  experiment  of  Prof.  Trowbridge  is,  however,  ap- 
parently opposed  to  such  an  inference.  It  appears 
that  Prof.  Trowbridge  cooled  a  magnet  that  had  not 
reached  a  constant  state  (that  is  to  say,  one  that  on 
cooling  would  not  have  completely  recovered  its  mag- 
netization on  cooling),  because,  after  the  magnet  had 
been  cooled  to  — 80°,  on  regaining  the  ordinary  tempera- 
ture it  had  lost  50  per  cent  of  its  original  magnetic 
moment.  Such  a  magnet  would  apparently  diminish 
in  magnetic  moment  on  cooling  and  heating  the  first 
time  the  action  was  examined,  but  a  repetition  of  the 
process  when  the  action  of  magnetization  and  tempera- 
ture were  strictly  reversible  might  lead  to  an  opposite 
conclusion.  To  settle  this  question,  a  series  of  experi- 
ments on  the  magnetic  moment  of  small  magnets 
cooled  to  — 182°  were  carried  out.  Small  magnets  from 
half  an  inch  to  an  inch  in  length  were  made  of  watch 


MAGNETISM.  35 

spring  or  steel  wire,  and  were  either  used  separately  or 
in  bundles.  ...  After  the  first  cooling  the  magnet  is 
allowed  to  regain  the  ordinary  temperature,  and  the 
operation  of  cooling  and  heating  is  repeated  three  or, 
four  tunes." 

In  summing  up  his  results  Prof.  Dewar  finds  that 
every  magnet  has  individual  characteristics  that  may 
either  result  in  no  change  on  cooh'ng  or  the  addition 
or  subtraction  of  from  12  to  24  per  cent  in  the  magnetic 
strength.  His  work  bears  directly  upon  the  molecular 
theory  of  magnetism.  Prof.  Ewing  has  also  shown 
how  magnetic  models  consisting  of  an  aggregation  of 
magnets  suitably  suspended  can  represent  most  of  the 
phenomena  of  magnetism,  irrespective  of  rotary  phe- 
nomena. 

Before  the  year  1800  the  compass  represented  to  the 
world  the  principal  use  of  magnetism ;  later  magnetism 
became  like  the  genius  which- Aladdin  called  into  exist- 
ence by  motion.  Furthermore,  the  force  of  attraction 
between  the  poles  of  two  magnets  was  considered  as  an 
action  at  a  distance,  without  the  intervention  of  any 
medium  between  the  poles.  "We  shall  see  that  the  great 
advances  in  our  knowledge  of  electricity  and  magnet- 
ism have  come  from  our  study  of  the  actions  in  the 
medium  between  two  magnetic  poles  or  two  electrified 
pith  balls. 

The  general  phenomena  of  magnetism  apart  from 
electro-magnetism  were  known  at  an  early  period,  and 
the  earth  was  recognised  as  a  great  magnet.  It  was 
observed  that  all  pieces  of  iron  and  steel  became  more 
or  less  magnetic  if  they  were  left  standing  at  an  angle 
with  the  horizontal  plane,  which  in  this  latitude  is  about 
73°.  A  poker,  for  instance,  suspended  by  a  fireplace 
will  generally  show  two  poles.  The  singular  fact  that 


36 


WHAT  IS  ELECTRICITY? 


there  is  a  limit  to  the  length  of  a  magnet  was  early  com- 
mented upon.  It  is  impossible  to  maintain  the  poles  of 
a  magnet  ten  feet  from  each  other  on  a  steel  bar  ;  in- 
termediate poles  will  generally  form,  which  are  called 
consequent  poles.  My  attention,  a  few  years  since,  was 
called  to  a  magnetic  motor  which  well  illustrates  the 
formation  of  consequent  poles.  An  inventor  claimed 
that  he  had  discovered  a  neu- 
tral line  in  the  space  near 
the  poles  of  a  horseshoe 
magnet.  His  motor  con- 
sisted of  an  armature  of  soft 
iron  (1ST  S,  Fig.  3)  on  the  end 
of  a  balance  arm.  The  other 
end  of  the  arm  vibrated  be- 
tween two  stops.  It  was 
claimed  that  when  the  proper 
rate  of  vibration  was  reached 
the  armature  would  move 
across  the  neutral  line  to  and 
fro,  the  neutral  line  acting 
as  a  species  of  cut-oil.  The 
existence  of  the  neutral  line 
was  shown  by  presenting 
small  tacks  to  a  bar  of  iron 
which  was  moved  in  front 
of  the  poles  of  the  magnet. 
As  the  bar  was  moved  away  from  the  poles  the  tacks 
dropped  off ;  but  in  continuing  the  motion  away  from 
the  magnet  another  position  was  found  in  which  the 
tacks  again  were  attracted  to  the  bar.  The  bar  appa- 
rently had  moved  through  a  neutral  line.  The  phe- 
nomenon, however,  was  caused  by  the  shifting  of  the 
consequent  poles,  due  to  the  changing  intensity  of  the 


FIG.  3. 


MAGNETISM.  37 

field,  and  had  no  real  existence.  It  is  needless  to  say 
that  the  motor  did  not  run  from  this  cause. 

For  many  years  there  was  no  substantial  change  in 
the  form  of  the  ship's  compass.  One  or  more  compara- 
tively heavy  bar  magnets  were  affixed  to  a  card  bearing 
the  points  of  the  compass,  and  the  card  was  pivoted  so 
that  its  graduations  should  pass  fixed  points  on  the  com- 
pass box.  Lord  Kelvin  made  a  great  improvement  in 
the  old  form  by  fixing  a  number  of  very  light  steel 
magnets  on  a  light  disk,  getting  in  this  way  strong  mag- 
netism, so  to  speak,  or  magnetic  moment  combined 
with  extreme  lightness  and  steadiness.  The  employ- 
ment of  iron  ships  has  made  it  necessary  to  compensate 
the  attraction  of  the  vessel  upon  the  compasses  by 
placing  steel  magnets  in  proper  positions  on  the  deck 
near  the  compass,  or  by  placing  a  compass  high  above 
the  deck,  and  by  means  of  its  indications  correcting  the 
lower  compasses.  It  is  necessary  also  to  swing  the 
ship  occasionally — that  is,  to  take  the  bearings  of  some 
point  on  the  shore  near  a  harbour  while  the  ship  is 
turned  completely  around.  In  this  way  the  attraction 
of  the  mass  of  the  ship  on  the  compass  when  sailing  on 
different  courses  is  ascertained. 

The  force  between  two  attracting  pith  balls  can  be 

„     m  m.      , 
represented  by  f  = — y-2  where  m  and  mt   are  the 

charges  on  the  pith  balls  and  r  is  the  distance  between 
them.  When  there  is  a  plate  of  glass  between  the 
charged  balls  the  force  is  very  much  diminished.  If  ~k 
is  a  -factor  depending  on  the  insulating  power  of  the 

glass,  the  force  is  F=-j- — ~ ;  that  is,  the  greater  the 

insulating  power  the  smaller  the  force.  In  a  similar 
manner  we  can  express  the  force  of  attraction  between 


38  WHAT  IS  ELECTRICITY? 


•  i  T~r  >         i      •  i   •    i  i 

two  magnetic  poles  as  J*  =  —  ~-^  in  which  m  and  mt 

represent  the  strength  of  the  poles  and  r  is  the  distance 
between  them.  If,  however,  the  poles  are  immersed  in 
a  solution  of  iron  the  attraction  between  them  would  be 

-,  ,      -7-,     in  m.   .         .          .  .        ,  .  , 

expressed  by  jp  =  -  r,  in  which  /j,  is  a  quantity  which 

depends  upon  the  medium.  If  /z.  is  large,  the  force  of 
attraction  is  small.  The  attraction,  for  instance,  be- 
tween two  magnets  placed  in  a  vessel  containing  a  salt 
of  iron  would  be  less  than  in  air. 

It  is  to  such  considerations  of  the  nature  of  the  sur- 
rounding medium  that  we  owe  the  advances  in  our 
knowledge  of  magnetism.  Previous  to  the  year  1800, 
I  have  said,  no  account  was  taken  of  the  surrounding 
media,  and  one  magnetic  pole  was  considered  to  act  upon 
another  as  if  it  were  an  action  at  a  distance  and  not  from 
point  to  point  in  the  medium  between  the  attracting  poles. 
It  is  well  to  consider  these  two  points  of  view  carefully, 
for  in  the  present  study  of  electricity  and  magnetism  we 
are  chiefly  occupied  with  a  study  of  what  goes  on  in  the 
medium  in  which  the  attracting  bodies  are  immersed. 
The  early  workers  in  magnetism  apparently  formed  no 
mental  picture  of  the  state  of  the  field  in  which  the  at- 
tracting bodies  were  placed.  They  had  not  formed  the 
conception  of  lines  of  force,  although  they  were  familiar 
with  the  arrangement  of  iron  filings  around  the  poles  of  a 
magnet.  The  defenders  of  the  theory  of  action  at  a  dis- 
tance did  not  carefully  examine  the  disturbance  of  a  mag- 
netic field  by  the  introduction  of  a  magnetic  pole,  whereas 
in  the  modern  theory  of  action  from  point  to  point  in  the 
medium  this  disturbance  is  fully  considered.  All  space 
around  the  earth  is  filled  with  lines  of  force,  which  crowd 
into  the  north  and  south  pole.  In  the  room  in  which 


MAGNETISM.  39 

the  reader  of  this  book  is  seated  these  lines  are  generally 
parallel  and  the  space  of  the  room  is  filled  with  them. 
If  there  is  an  iron  article  in  the  room  they  tend  to  crowd 
through  the  iron,  finding  it  a  better  conductor  than  the 
air.  We  say,  therefore,  that  there  is  a  flow  of  magnetic 
induction  through  the  iron.  We  can  think  of  magnetic 
induction  as  the  accumulation  of  the  lines  of  force  in 
the  space  occupied  by  the  iron.  To  obtain  a  space  any- 
where on  the  earth's  surface  free  from  lines  of  force  we 
must  employ  a  thick- walled  hollow  sphere  of  iron.  In 
the  space  inside  the  sphere  there  will  be  no  lines  of  force. 
The  space  has  been,  so  to  speak,  swept  free  of  lines  of 
force,  and  there  is  a  flow  of  induction  through  the  walls  of 
iron.  A  compass  inside  such  a  sphere  would  no  longer 
be  influenced  by  the  poles  of  the  earth.  An  examina- 
tion of  the  terminology  of  a  subject  often  will  show  the 
trend  of  the  subject.  We  speak  to-day  of  the  flow  or 
flux  of  magnetic  induction,  and  we  picture  to  ourselves 
this  flow  as  taking  place  from  one  pole  of  a  magnet  to 
the  other  through  the  air.  We  also  speak  of  closed 
magnetic  circuits,  which  we  obtain  to  a  great  extent 
when  we  put  an  armature  on  a  horseshoe  magnet.  The 
flow  of  induction  takes  place  then  through  the  armature, 
and  not  through  the  air.  A  compass  is  not  affected  by 
a  closed  magnetic  circuit,  for  no  lines  of  force  escape  to 
flow  through  its  steel.  We  also  speak  of  the  resistance 
of  the  air  to  the  flow  of  magnetic  induction.  It  is  harder 
to  force  the  flow  of  magnetic  induction  through  air  than 
through  iron  or  steel.  If,  for  instance,  we  should  cut  a 
horseshoe  magnet  in  two  at  its  bend  and  then  put  the 
ends  together  again,  it  can  not  be  made  as  strong  as  it 
was  originally ;  although  the  joint  may  be  made  very 
perfect,  the  thin  layer  of  air  opposes  a  resistance  to  the 
flow  of  induction.  We  also  speak  of  the  magneto-mo- 


4-0  WHAT  IS  ELECTRICITY? 

tive  force  which  establishes  the  flow  of  induction,  much 
as  we  speak  of  the  electro-motive  force  which  estab- 
lishes a  current  of  electricity.  In  a  certain  sense,  there- 
fore, we  can  regard  a  magnet  as  a  little  battery.  It 
has  a  magneto-motive  force.  It  has  a  circuit  with  a 
certain  resistance.  It  has  a  flow  analogous  to  a  current. 

Although  this  terminology  shows  that  we  are  study- 
ing the  action  from  point  to  point  in  the  substance  of  a 
magnet  and  in  the  space  around  it,  we  must  not  con- 
clude that  we  have  evidence  of  a  flow  of  a  magnetic 
fluid  or  the  flow  of  the  ether  of  space  around  and 
through  the  substance  of  a. magnet.  We  shall  see,  also, 
that  we  have  no  evidence  of  an  actual  flow  of  electricity 
in  the  case  of  an  electrical  current. 

It  is  often  stated  that  certain  persons  have  detected 
a  peculiar  effect  when  the  poles  of  a  magnet  are  passed 
over  their  bodies,  and  that  they  have  seen  lambent 
flames  proceeding  in  the  dark  from  the  poles  of  a  mag- 
net. The  reader  curious  in  this  matter  will  find  many 
observations  of  these  alleged  phenomena  in  a  book  en- 
titled The  Odic  Force,  by  Baron  Keichenbach.*  It  has 
been  found,  however,  that  wooden  bars  painted  to  re- 
semble magnets  produce  the  same  effect  as  the  actual 
magnets.  The  effect  of  very  powerful  electro -magnets 
on  the  human  body  has  been  tried  recently.  With  the 
head  placed  between  the  poles  of  an  electro-magnet 
sufficiently  strong  to  lift  tons  of  weight  one  can  carry 
on  mental  calculations  with  perfect  ease,  and  a  genius 
undisturbed  by  the  milieu  doubtless  could  write  a  sonnet 
to  magnetism.  The  human  nervous  system  does  not 
appear  to  be  sensitive  to  magnetic  force. 


*  Edited  by  John  Ashburner,  M.  D.    Partridge  &  Brittan,  New 
York,  1855. 


MAGNETISM.  4J 

On  the  other  hand,  all  bodies  may  be  considered 
more  or  less  magnetic.  The  oxygen  of  the  air  is  highly 
so  compared  with  other  gases.  If  the  air  were  deprived 
of  its  oxygen  magnetic  poles  would  attract  each  other 
more  strongly.  The  early  electricians  divided  all  bodies 
into  two  classes,  paramagnetic  and  diamagnetic.  Iron 
and  steel,  nickel  and  cobalt,  are  paramagnetic  bodies. 
Bismuth  is  an  example  of  a  diamagnetic  body.  A  bar 
of  bismuth  will  place  itself  at  right  angles  to  the  line 
between  a  north  and  a  south  pole,  instead  of  in  this 
line,  as  a  bar  of  iron  or  steel  would  do. 

In  the  case  of  electricity  and  magnetism  the  medium 
between  conductors  of  electricity  or  between  magnets 
plays  an  important  part  in  the  phenomena  of  what  is 
called  induction — in  other  words,  the  phenomena  which 
result  from  the  stresses  called  forth  in  the  medium  by 
the  electro-magnetic  waves.  In  gravitation,  the  medium, 
however,  does  not  affect  the  attraction  between  the 
bodies.  A  plate  of  iron  or  a  plate  of  paraffin  between 
two  attracting  spheres  does  not  affect  their  pull  upon 
each  other.  There  is  also  no  evidence  of  a  directive 
action  of  gravitation — that  is,  an  elongated  cylinder 
does  not  tend  to  place  its  longer  axis  vertical  on  the 
earth's  surface. 

Maxwell  shows  that  "  if  we  assume  that  the  medium 
is  in  a  state  of  stress,  consisting  of  tension  along  the 
lines  of  force  and  pressure  in  all  directions  at  right 
angles  to  the  lines  of  force,  the  tension  and  the  pressure 
being  equal  in  numerical  value  and  proportional  to  the 
square  of  the  intensity  of  the  field  at  the  given  point, 
the  observed  electrostatic  and  electro-magnetic  forces 
will  be  completely  determined.  ...  A  path  is  now  open 
by  which  we  may  trace  to  the  action  of  a  medium  all 
forces  which,  like  the  electric  and  magnetic  forces,  vary 


42  WHAT  IS  ELECTRICITY? 

inversely  as  the  square  of  the  distance,  and  are  attractive 
between  bodies  of  different  names,  and  repulsive  be- 
tween bodies  of  the  same  names." 

Gravitation  differs  from  electrical  and  magnetic 
attractions  in  that  there  is  no  difference  of  sign,  no  posi- 
tive and  negative.  We  must  therefore  assume  a  differ- 
ent kind  of  stress.  "  We  must  suppose  that  there  is  a 
pressure  in  the  direction  of  the  lines  of  force,  combined 
with  a  tension  in  all  directions  at  right  angles  to  the 
lines  of  force.  Such  a  stress 
would  no  doubt  account  for 
the  observed  effects  of  gravita- 
tion." 

Prof.  C.  A.  Bjerknes,*  in 
a   very  suggestive    paper   on 
B  Hydrodynamical      Analogies, 

has  shown  that  magnetic  and 
electrical   attractions  and   re- 
JL  pulsions   can   be  imitated  by 

'  ^  -'-NX  giving  fluids  a  pulsating  or 
)  oscillating  movement.  By  the 
term  pulsation  is  meant  a  pe- 
riodical change  of  volume ;  by 
that  of  oscillation,  a  change 
of  length.  Bjerknes'  pulsator  consists  of  a  little  tam- 
bour or  drum  (A,  Fig.  4),  covered  on  both  ends  with 
membrane.  The  drum,  A,  is  connected  by  rubber  tub- 
ing to  an  alternating  air  pump,  which  communicates  air 
pulsations  to  the  drum.  The  oscillator,  B,  consists  of  a 
sphere  supported  on  a  little  lever ;  this  lever  can  be  set 
in  vibration  by  the  alternating  air  pump,  communica- 
tion being  obtained  by  means  of  a  tube.  The  pul- 

*  Kepertoriutn  dcr  Physik,  xis,  1883. 


MAGNETISM.  43 

sators  and  the  oscillators  were  mounted  on  suitable 
balance  arms,  and  were  immersed  in  fluids.  Thus,  on 
working  the  alternating  pump 
pulsating  or  oscillatory  move- 
ments could  be  communicated 
to  these  fluids. 

When  two  pulsators  are 
brought  near  each  other  and 
when  the  pulsations  are  in 
the  same  phase  there  is  an  at- 
traction (Fig.  5).  On  the  FlG  5 
other  hand,  if  the  pulsations 

are  discordant — that  is,  in  opposite  phases — repulsion 
occurs.  This  is  the  exact  opposite  to  what  takes  place 
in  magnetism  where  like  poles  repel  and  unlike  attract. 
Experiments  were  next  tried  with  a  combination  of  pul- 
sators and  oscillators.  If  an  oscillating  sphere  was 
brought  near  a  pulsator,  attraction  or  repulsion  could 
be  obtained — attraction,  if  the  oscillating  sphere  ap- 
proached the  pulsator  at  the  instant  of  its  dilatation,  or 
repulsion  if  the  sphere  was  removed  from  the  pulsator 
hi  the  time  of  its  dilatation.  Here  we  obviously  have 
the  means  of  drawing  many  interesting  analogies  be- 
tween the  motion  of  fluids  and  the  phenomena  of  elec- 
tricity and  magnetism.  Bjerknes  shows  that  the  phe- 
nomena of  induction,  temporary  and  permanent  magnet- 
ism, and  the  phenomenon  of  the  magnetic  field  can  be 
imitated  by  his  hydrodynamic  apparatus. 

Although  analogies  drawn  from  the  action  of  fluids 
are  highly  interesting,  they  remain  at  the  present  time 
in  the  subject  of  electro-magnetism  merely  analogies. 
Fluids  are  gross  substances,  and  their  molecular  group- 
ing and  attractions  are  of  such  magnitude  that  we  can 
not  reason  safely  from  actions  in  them  to  actions  in  a 


44  WHAT  IS  ELECTRICITY! 

highly  attenuated  fluid  or  medium,  such  as  that  which 
is  supposed  to  exist  in  the  space  about  electrical  con- 
ductors and  magnets.  Wave  motions  in  the  ether  are 
supposed  to  take  place  around  the  atoms  or  molecules 
of  bodies.  These  wave  motions  are  handicapped,  so  to 
speak,  by  the  presence  of  such  bodies  and  by  the  waves 
which  the  vibrations  of  the  bodies  impress  on  the 
ether.  Bjerknes's  experiments  have  led  many  philoso- 
phers to  speculate  upon  the  possibility  that  the  pulsation 
or  oscillatory  movements  of  an  ether  might  account  for 
the  phenomena  of  gravitation  and  of  attraction  or  re- 
pulsion in  general. 

Lord  Kelvin,  in  an  address  on  terrestrial  magnetism, 
remarks,  "  I  find  it  unimaginable  but  that  terrestrial 
magnetism  is  due  to  the  greatness  and  rotation  of  the 
earth  "  ;  and  says  further  in  regard  to  the  hypothesis  that 
magnetic  disturbances  are  caused  by  the  sun  acting  as  a 
variable  magnet :  "  In  eight  hours  of  a  not  very  severe 
magnetic  storm  as  much  work  must  have  been  done  by 
the  sun  in  sending  magnetic  waves  out  in  all  direc- 
tions through  space  as  he  actually  does  in  four  months 
of  his  regular  heat  and  light.  This  result,  it  seems  to 
me,  is  absolutely  conclusive  against  the  supposition  that 
terrestrial  magnetic  storms  are  due  to  magnetic  action 
of  the  sun ;  or  to  any  kind  of  dynamical  action  taking 
place  within  the  sun,  or  in  connection  with  hurricanes 
in  his  atmosphere,  or  anywhere  near  the  sun  outside. 
It  seems  as  if  we  may  also  be  forced  to  conclude  that 
the  supposed  connection  between  magnetic  storms  and 
sun  spots  is  unreal,  and  that  the  seeming  agreement 
between  the  periods  has  been  a  mere  coincidence."  * 


*  Popular  Lectures  and  Addresses,  by  Sir  William  Thomson  (Lord 
Kelvin). 


CHAPTER  IV. 

THE   ELECTRIC   CURRENT. 

A  DISTINGUISHED  American  lecturer  once  delivered 
a  brilliant  course  of  lectures  on  the  Lost  Arts,  in  which 
he  showed  that  the  ancients  possessed  a  knowledge  of 
many  processes  now  lost  to  the  world,  and  he  intimated 
that  the  nation  that  could  transport  immense  obelisks 
over  great  distances,  and  build  the  pyramids,  might  also 
have  had  a  knowledge  of  science  far  beyond  what  his- 
torical investigation  has  revealed  to  us.  It  may  seem 
unsafe  to  assert  positively  that  the  ancients  knew  noth- 
ing of  electricity  save  what  is  manifested  by  the  rub- 
bing of  amber ;  for  modern  excavations  are  continually 
increasing  our  knowledge  of  ancient  life.  Not  the 
slightest  evidence,  however,  exists  that  a  knowledge  of 
the  art  of  covering  a  wire  uniformly  with  silk,  cotton, 
or  other  nonmetallic  or  insulating  substance  existed  be- 
fore the  time  of  Benjamin  Franklin. 

Without  insulated  wire  no  progress  was  possible  in 
electricity,  for  the  electro-magnet,  by  means  of  which 
we  transmit  telegraph  messages,  propel  cars,  and,  in 
short,  accomplish  most  of  the  modern  wonderful  results 
in  electricity,  consists  merely  of  a  spool  of  cotton-  or 
silk-covered  wire  placed  upon  an  iron  core.  The  rapid 
development  of  electrical  science  during  the  past  thirty 
years  is  largely  due  to  the  facility  with  which  a  piece  of 


46  WHAT  IS  ELECTRICITY? 

iron  can  be  wound  with  covered  wire.  Prof.  Joseph 
Henry,  whose  electrical  researches  led  to  the  invention 
of  the  telegraph,  spent  months  in  wrapping  wire  with 
strips  of  cloth,  in  order  to  make  the  magnets  by  means 
of  which  he  showed  the  possibility  of  transmitting 
signals  to  a  distance  by  electricity.  To-day  far  more 
powerful  magnets  than  he  constructed  can  be  made  in 
an  hour. 

This  wonderful  something  which  we  call  electricity 
circulating  around  coils  of  covered  wire  makes  an  iron 
core  a  magnet.  The  oftener  we  make  it  flow  around 
the  iron,  or,  within  certain  limits,  the  more  turns  of 
wire  we  put  upon  our  spool,  keeping  the  strength  of 
the  electric  current  constant,  the  stronger  the  magnet. 
Now,  in  considering  this  phenomenon  we  are  led  to  the 
remarkable  fact  that  a  thin  covering  of  silk  or  cotton 
can  prevent  the  electric  current,  such  as  is  used  to  pro- 
pel cars,  from  being  conducted  from  one  layer  of  wire 
on  the  spool  to  the  next.  The  thinnest  sheet  of  paper 
placed  under  the  trolley  of  an  electric  car  will  stop  the 
car,  provided  that  it  is  not  punctured  by  the  mechan- 
ical pressure.  Cut  a  copper  wire  carrying  an  electrical 
current,  file  the  ends  square,  place  a  sheet  of  writing 
paper  between  the  ends  and  press  them  together :  the 
current  which  was  transmitting  thousands  of  horse 
power  is  stopped;  it  is  incapable  of  passing  through 
the  insulating  substance  of  the  paper.  Here  we  are 
brought  to  a  realizing  sense  of  one  of  the  chief  pecul- 
iarities of  the  method  of  transmitting  power  by  elec- 
tricity. No  other  agency  for  transmitting  power  can 
be  stopped  by  such  slight  obstacles  as  electricity.  A 
sheet  of  writing  paper  placed  across  a  tube  conveying 
compressed  air  would  be  instantly  ruptured.  It  would 
take  a  wall  of  steel  at  least  an  inch  thick  to  stand  the 


THE   ELECTRIC  CURRENT.  47 

pressure  of  steam  which  is  driving  a  10,000-horse-power 
engine.  A  thin  layer  of  dirt  beneath  the  wheels  of  an 
electric  car  can  prevent  the  current  which  propels  the 
car  from  passing  to  the  rails,  and  thus  back  to  the  power 
house. 

Another  striking  difference  between  what  we  call  a 
steady  electrical  current  in  a  wire  and  steam  or  air  at 
high  pressure  in  a  pipe  is  the  absolute  stillness  which 
marks  its  sudden  passage  from  one  wire  to  another  one 
of  larger  diameter.  In  the  case  of  air  or  steam,  this 
sudden  passage  from  a  small  conductor  or  pipe  to  a 
larger  receptacle  would  be  accompanied  by  a  whistle  or 
roar.  There  is  no  sound  heard  in  a  wire  when  an  elec- 
tric current  is  suddenly  established  in  it.  One  can 
handle  the  wire  carrying  thousands  of  horse  power 
without  experiencing  the  slightest  sensation,  and  birds 
can  sit  on  such  a  wire  with  the  safety  that  they  can  rest 
on  a  dead  limb  of  a  tree.  When  the  wire  is  broken, 
however,  there  is  a  blinding  flash  of  light  and  a  loud 
report — a  miniature  thunderstorm.  The  only  analogy 
between  our  pipes  conveying  air  or  steam  at  high  pres- 
sure and  our  wire  carrying  an  electrical  current  is  in  the 
heat  which  we  can  perceive  on  both  the  pipes  and  the 
wire.  This  analogy  is  a  valuable  one,  which  we  must 
bear  in  mind  as  we  endeavour  to  discover  what  electrici- 
ty is.  The  transmission  of  compressed  air  or  of  steam 
by  pipes  is  not  affected  by  the  presence  or  the  motion 
of  surrounding  objects  outside  the  pipes;  a  compass 
needle  remains  quiescent.  The  neighbouring  pipes  are 
not  attracted  or  repelled  from  each  other.  This  is  not 
true  in  the  case  of  the  electrical  transmission  of  power. 
A  compass  needle  instantly  points  to  the  wire  carrying 
the  current ;  and  iron  dust  or  filings  gather  around  the 
wire.  Two  neighbouring  wires  carrying  currents  are 


48  WHAT  IS  ELECTRICITY! 

attracted  to  each  other  if  the  currents  are  going  in  the 
same  direction,  or  repelled  if  they  are  going  in  opposite 
directions.  The  quick  removal  of  one  of  these  wires 
from  its  proximity  to  the  other  makes  the  currents  in  the 
wires  throb,  just  as  a  change  in  the  pressure  of  the  air 
or  steam  in  a  pipe  would  cause  a  fluctuation  in  the  trans- 
mission of  power. 

The  pressure  of  compressed  air  or  steam  is  not 
sensibly  affected  by  a  rapid  motion  of  the  pipe  contain- 
ing it ;  for  instance,  we  could  quickly  revolve  a  hollow 
ring  like  a  top  about  a  diameter,  allowing  steam  at  high 
pressure  to  pass  into  the  ring  through  a  suitable  valve 
at  one  end  of  the  diameter  and  out  of  the  ring  at  the 
other.  The  whirling  motion  of  such  a  moving  ring 
would  not  sensibly  affect  the  air  or  steam  in  the  pipe. 
If  a  current  of  electricity,  however,  should  enter  and 
leave  a  ring  made  of  wire  it  could  be  very  much  af- 
fected by  the  rapid  motion  of  the  ring.  The  quick 
rolling  of  a  steamship  does  not  modify  the  pressure  of 
the  steam  in  the  pipes  conveying  it  to  the  engines ;  it 
does,  however,  affect  the  electric  current  which  is  light- 
ing the  steamship,  although  the  effect  in  the  case  of  the 
comparatively  slow  motion  of  the  steamship  is  small. 
When  a  loop  or  ring  of  copper  wire  carrying  a  current 
is  revolved  many  times  a  minute  before  the  pole  of  a 
powerful  magnet  a  fluctuating  effect  can  be  produced 
which  would  be  sufficient  to  make  the  electric  lights  of 
the  steamship  fed  by  this  current  blink  painfully.  The 
pole  of  the  earth  exerts  a  similar  effect  on  moving  wires 
carrying  currents. 

The  mysterious  something  which  we  call  an  electric 
current  is  therefore  influenced  by  the  motion  of  sur- 
rounding objects  and  by  the  motion  of  its  own  path ;  it 
has  little  in  common  with  the  compressed  air  or  steam 


THE  ELECTRIC   CURRENT.  49 

which  is  conveying  an  equal  amount  of  horse  power, 
except  in  the  evidence  of  heat  along  its  path. 

We  have  said  that  the  only  peculiarity  which  a  pipe 
conveying  steam  or  compressed  air  possesses  in  common 
with  a  copper  wire  conveying  an  electric  current  resides 
in  the  development  of  heat  along  the  conductor.  If  we 
should  narrow  the  bore  of  a  steam  pipe  to  the  size  of  a 
knitting  needle,  we  should  greatly  restrict  the  flow  of 
steam  through  the  pipe  and  reduce  the  amount  of  horse 
power  that  was  previously  transmitted.  The  same  is 
true  of  a  water  pipe :  the  flow  of  water  is  greatly  im- 
peded by  the  constriction  of  the  pipe.  In  the  case  of 
steam  and  compressed  air  there  is  not  a  notable  or  great 
increase  of  heat  when  the  bore  of  the  conducting  pipe 
is  made  smaller.  "With  electricity,  however,  there  is  a 
remarkable  increase  of  heat  when  the  conductor  is 
greatly  reduced  in  diameter.  This  can  be  seen  by  ex- 
amining the  electric  light  in  our  houses.  The  main  con- 
ductors are  of  large  size  and  of  pure  copper,  while  the 
filament  of  the  lamps  is  fine,  and  is  of  carbon.  The 
narrowing  of  the  electric  conductor,  therefore,  leads  to 
a  great  development  of  heat  and  light ;  and  here  we 
must  bear  in  mind  that  the  only  difference  between 
heat  and  light  consists  in  wave  length ;  the  heat  waves 
are  much  longer  than  the  light  waves.  As  we  continue 
our  study  we  shall  find  also  that  the  only  difference 
between  light,  heat,  and  electricity  is  in  the  length  of 
waves. 

Our  analogies  between  pipes  conveying  horse  power 
by  means  of  steam  or  compressed  air  and  wires  carrying 
electricity,  we  have  seen,  do  not  lead  us  far.  There  is 
an  evidence  of  pressure  in  such  pipes,  and  also  on  con- 
ductors carrying  electric  currents.  There  is  a  develop- 
ment of  heat  on  both  pipes  and  on  electric  conductors, 


50  WHAT  IS  ELECTRICITY! 

but  this  development  is  much  greater  in  the  case  of 
electricity.  Here  our  analogy  stops  short.  The  steam 
pipe  exerts  no  influence  outside  itself  to  attract  or  repel 
other  pipes.  Its  effect  on  a  magnet  is  the  same  whether 
the  steam  flows  through  it  or  not ;  it  acts  in  both  cases 
like  a  piece  of  iron ;  it  is  not  aroused  to  the  exertion  of 
a  tremendous  power'  on  a  neighbouring  magnet  or  on  a 
neighbouring  conductor  carrying  a  current  when  steam 
rushes  through  it ;  it  is  dead  to  changing  magnetic  influ- 
ences. But  the  electrical  current  hi  a  conductor  seems 
to  exert  a  mysterious  influence  on  all  neighbouring  ob- 
jects, even  on  the  surrounding  air.  This  influence  is 
especially  marked  on  magnets  and  on  neighbouring  elec- 
tric currents.  The  western  end  of  the  Jefferson  Phys- 
ical Laboratory  of  Harvard  University  was  constructed 
with  especial  reference  to  freedom  from  magnetic  dis- 
turbances, in  order  that  delicate  experiments  in  elec- 
tricity and  magnetism  might  be  conducted  there.  All 
the  steam  pipes  and  gas  pipes  were  constructed  of  brass, 
and  no  iron  nails  were  used  in  the  flooring.  This  con- 
struction was  costly ;  but  it  has  been  made  useless  by  the 
presence  of  the  overhead  wire  of  an  electric  road.  Every 
time  an  electric  car  passes  a  fluctuation  of  the  electric 
current  is  caused  on  the  overhead'  wire ;  all  the  delicate 
magnetic  instruments  in  the  laboratory  throb  in  unison 
with  the  electric  disturbance,  and  this,  too,  at  a  distance 
of  at  least  400  feet.  In  scientifically  enlightened  Ger- 
many the  electric  cars  are  not  permitted  to  run  near  the 
great  physical  laboratory  at  Charlottenburg,  near  Berlin. 
We  see,  therefore,  that  the  mysterious  phenomenon 
we  call  an  electric  current  is  extremely  versatile.  It 
more  nearly  resembles  the  nervous  action  of  man  than 
any  other  influence  with  which  we  have  to  deal.  We 
can  not,  however,  speak  of  a  nerve  current,  and  we 


THE   ELECTRIC  CURRENT.  51 

shall  see  later  that  strictly  considered  we  ought  not  to 
speak  of  an  electric  current,  for  there  is  but  little  evi- 
dence that  there  is  a  flow  of  electricity  in  a  wire  which 
we  ordinarily  say  conveys  a  current.  I  shall  continue 
to  use  the  term  electric  current  only  in  a  popular  sense. 
Before  we  leave  our  consideration  of  what  may  be 
called  the  palpable  phenomena  of  the  electric  current, 
and  before  we  enter  upon  a  study  of  the  source  and 
generation  of  this  current  in  a  conductor,  let  us  dwell  a 
little  longer  on  the  development  of  heat  produced  by 
electricity  on  conductors.  In  our  cities  it  often  happens 
that  the  overhead  wire  of  the  electric  railroads  falls  to 
the  ground.  When  it  touches  the  rails  the  wire  is 
raised  to  a  white  heat,  and  contorts  and  twists  like  a 
liery  serpent.  With  a  stick  or  a  thickly  folded  news- 
paper it  is  perfectly  safe  to  lift  the  wire  from  the  rails 
and  thus  stop  the  development  of  heat.  If  it  were  not 
for  the  mere  heat,  one  could  take  hold  of  the  wire  with 
a  handkerchief  and  ran  no  danger  of  an  electric  shock ; 
for  we  have  said  that  a  piece  of  paper  is  sufficient  to 
stop  the  flow  of  an  electric  current  which  represents 
many  thousand  horse  power.  A  general  knowledge  of 
the  impunity  with  which  we  can  handle  wires  carrying 
strong  currents,  if  we  seize  them  properly — that  is,  with  a 
folded  handkerchief  or  with  a  folded  newspaper — would 
often  prevent  needless  terror  and  save  life.  From  our 
consideration,  therefore,  of  the  development  of  electric 
light  when  a  conductor  is  reduced  to  the  size  of  a  thread, 
and  from  the  study  of  the  phenomenon  of  the  great 
heating  of  the  overhead  wire  of  an  electric  road  when 
it  falls  to  the  ground  and  touches  the  rails,  thus  gaming 
a  quick  return  to  the  power  house  where  the  source  of 
electricity  is  situated,  we  are  led  to  two  conclusions : 
First,  that  a  narrowing  or  constricting  of  the  conductor, 
5 


52  WHAT  IS  ELECTRICITY! 

or,  in  other  words,  an  increase  of  resistance,  leads  to  a 
great  development  of  heat  and  light ;  second,  that  a 
shortening  of  the  path  or  a  lessening  of  resistance  also 
leads  to  a  great  development  of  heat.  These  are  ap- 
parently contradictory  conclusions.  The  contradiction, 
however,  is  explained  when  we  discover  by  suitable 
measuring  instruments  that,  whenever  we  diminish  the 
diameter  of  an  electric  conductor  in  order  to  produce 
light  or  heat,  we  reduce  the  total  flow  of  electricity 
through  the  entire  electric  circuit ;  less  goes  in  general 
through  the  large  wires  leading  to  our  electric  lamps 
when  they  are  lighted  than  when  they  are  not  lighted. 
The  heating  of  an  electric  conductor  is  proportional  to 
the  square  of  the  strength  of  the  current  which  is 'flow- 
ing ;  that  is,  if  the  strength  of  the  current  is  doubled, 
the  amount  of  heat  developed  on  the  conductor  is  quad- 
rupled. We  can  see,  therefore,  why  the  large  overhead 
wire  of  an  electric  railroad  is  intensely  heated  when  it 
falls  from  its  guard  wires  and  touches  the  rails  of  the 
road.  The  current  does  not  encounter  the  resistance  of 
the  overhead  wire  beyond  the  point  where  it  touches  the 
rails,  as  it  usually  does  when  it  is  in  its  normal  position. 
This  portion  is  cut  out  by  the  fall  of  the  wire,  and  the 
entire  current  rushes  through  the  wire  between  the  feed- 
ing wires  and  the  spot  where  the  broken  wire  touches  the 
ground  and  forms  a  powerful  electric  arc  at  the  latter 
place.  If  one  should  notice  the  character  of  the  over- 
head wire  now  used  on  electric  railroads,  it  will  be  found 
that  it  is  of  copper  of  nearly  the  diameter  of  a  small 
lead  pencil.  Such  a  wire  opposes  very  little  resistance 
per  foot  to  the  flow  of  the  electric  current  which  is  used 
to  propel  the  cars.  Its  resistance,  however,  becomes 
very  appreciable  over  long  distances,  and  this  resistance 
is  one  of  the  reasons  why  electrical  power  can  not  be 


THE  ELECTRIC   CURRENT.  53 

transmitted  economically  farther  than  five  or  six  miles 
by  a  steady  current.  In  order  to  increase  this  distance 
we  must  increase  the  size  of  our  copper  conductor  to 
diminish  the  resistance,  and  the  cost  of  copper  speedily 
sets  a  limit  to  our  endeavours.  We  shall  see,  however, 
as  we  continue  our  study,  that  with  a  to-and-f  ro  or  alter- 
nating current  of  electricity — that  is,  an  unsteady  cur- 
rent— we  can  use  a  smaller  copper  wire,  and  thus  trans- 
mit electrical  power  over  hundreds  of  miles. 

The  resistance  of  copper  and  silver  to  the  steady 
electric  current  is  less  than  that  of  any  other  metals. 
The  resistance  of  iron  is  at  least  six  times  that  of  cop- 
per. A  veiy  slight  impurity  hi  copper  wire  exerts  a 
marked  increase  of  resistance.  The  use  of  a  98-per-cent 
copper  wire  in  the  place  of  a  97-per-cent  copper  wire 
on  a  long-distance  telephone  circuit  between  Boston  and 
Chicago  would  mean  a  great  saving  of  copper,  and 
therefore  of  money.  Twenty-five  years  ago  any  speci- 
men of  copper  wire  would  vary  in  resistance  in  differ- 
ent portions  of  its  length  by  at  least  1  per  cent,  and 
sometimes  more.  To-day  it  is  difficult  to  detect  any 
variation  in  resistance  in  hundreds  of  feet  of  copper 
wire  which  is  furnished  by  manufacturers.  This  per- 
fection of  material  is  due  to  the  demand  for  good  con- 
ductors by  electricians.  The  production  of  copper  has 
now  become  of  more  importance  to  the  world  than  the 
production  of  silver  ;  this  is  largely  due  to  the  remark- 
able practical  developments  of  electricity. 

An  interesting  application  of  the  phenomenon  of 
heating  due  to  increased  electrical  resistance  is  made  in 
surgery.  An  electrical  current  is  conducted  through  a 
fine  platinum  wire  which  is  raised  to  a  white  heat  by  the 
current.  This  localized  heat  can  be  used  to  cauterize 
portions  of  the  body  to  which  heat  could  not  hitherto 


54:  WHAT  IS  ELECTRICITY? 

be  applied ;  for  instance,  in  the  throat  and  in  the 
nasal  passages.  On  certain  electric  roads,  also,  the  cars 
are  heated  by  coils  of  iron  wire  through  which  an  elec- 
tric current  passes.  The  heat  is  due  to  the  resistance  of 
the  iron  wire.  It  is  thought,  indeed,  by  some  that  all 
the  culinary  operations  in  our  houses  in  time  will  be  per- 
formed by  electricity.  On  a  gridiron  of  wire  rendered 
red-hot  by  an  electric  current  a  beefsteak  can  be  readily 
cooked.  Water  can  be  heated  in  receptacles  in  which 
are  placed  coils  of  iron  wire ;  and  the  great  advantage 
of  such  applications  of  electricity  to  the  operations  of 
domestic  cookery  are  similar  to  those  in  its  applications 
to  surgery  :  the  heat  can  be  applied  exactly  at  the  spot 
where  it  is  most  needed. 

When  the  dynamo  machine  became  an  efficient 
source  of  electrical  currents  men's  minds  were  immedi- 
ately turned  to  the  problem  of  what  was  called  the  sub- 
division of  the  electric  light,  or,  in  more  appropriate 
words,  the  problem  of  obtaining  a  number  of  small  in- 
candescent lights  from  a  given  constant  electro -motive 
force  and  a  given  current.  It  was  an  easy  matter  to 
obtain  an  arc  light  between  carbon  points,  but  this  light 
was  suitable  only  for  street  lighting,  and  could  not  be 
made  to  contribute  to  the  comfort  of  humanity  in  each 
home.  It  was  early  recognised  that  whoever  could  aid 
in  solving  this  great  problem  would  render  mankind  a 
service.  As  we  have  said,  many  minds  were  at  work 
upon  this  problem  and  partial  solutions  were  obtained. 
It  was  soon  perceived  that  the  small  electric  light,  if  it 
were  invented,  must  be  obtained  by  raising  a  fine  metallic 
wire  or  a  carbon  filament  to  a  high  state  of  incandes- 
cence. At  first,  fine  platinum  wire  was  selected  for  this 
purpose,  since  its  high  degree  of  infusibility  seemed  to 
render  it  suitable ;  it  would  not  burn  out,  so  to  speak, 


THE  ELECTRIC   CURRENT.  55 

under  the  electric  heating.  It  was  found,  however,  that 
its  state  of  incandescence  was  variable,  and  that  even 
when  the  platinum  loop  was  placed  in  a  little  globe 
from  which  the  air  was  exhausted  this  inconstancy  of 
action  manifested  itself,  and  no  reliance  could  be  placed 
upon  the  platinum  incandescent  electric  lamp. 

When,  however,  a  suitable  filament  of  carbon  was 
substituted  for  the  platinum  wire  in  a  globe  from  which 
the  air  had  been  exhausted,  a  lamp  was  obtained  which 
would  burn  from  three  hundred  to  six  hundred  hours. 
To  maintain  a  number  of  such  lamps  on  an  electric  cir- 
cuit, it  was  speedily  discovered  that  they  must  all  re- 
ceive the  same  electric  pressure,  so  to  speak — that  is, 
the  current  must  be  urged  through  them  by  the  same 
electro-motive  force.  Moreover,  each  lamp  required 
the  same  amount  of  current  to  raise  it  under  the  given 
electric  pressure  to  the  same  candle  power.  This  could 
not  be  accomplished  by  placing  the  lamps  one  after  an- 
other on  the  same  wire,  for  the  resistance  offered  by 
each  lamp  diminished  the  current.  Two  or  three  lamps 
placed  in  this  manner  were  sufficient  to  prevent  a  great 
dynamo,  capable  of  manifesting  an  energy  equivalent 
to  many  hundred  horse  power,  from  exhibiting  this 
energy  along  the  electric  circuit.  A  simple  application 
of  KirchhofFs  laws  soon  solved  this  difficulty.  If  many 
paths  are  offered  to  the  electric  current,  it  divides  itself 
in  a  proportional  manner  among  these  paths ;  for  in- 
stance, if  we  should  consider  the  meridian  lines  of  the 
earth  as  electric  wires  meeting  at  the  poles,  and  we 
should  lead  an  electric  current  into  these  wires  at  the 
south  pole  of  the  earth  and  out  at  the  north  pole,  the 
current  would  divide  itself  proportionately  on  the  meri- 
dian wires.  Each  meridian  wire  would  receive  the  same 
amount  of  current,  and  each  subdivision  of  current 


56  WHAT  IS  ELECTRICITY  f 

would  be  under  the  same  difference  of  electric  pressure 
— namely,  the  difference  of  pressure  at  the  two  poles. 
A  little  lamp,  therefore,  placed  on  any  wire  represented 
by  a  meridian  would  glow  with  the  same  brilliancy  as 
that  of  its  neighbour  on  another  meridian ;  and,  further- 
more, according  to  Kirchhoffs  laws,  the  resistance  op- 
posed to  the  electric  flow  by  the  lamps  would  be  enor- 
mously diminished  by  this  arrangement,  which  is  termed 
the  multiple  circuit.  We  can  obtain  a  conception  of  a 
multiple  circuit  and  its  effect  hi  diminishing  resistance 
by  considering  the  flow  of  water  through  pipes.  If  we 
narrow  the  diameter  of  a  pipe  we  increase  the  resistance 
to  the  flow  of  water ;  if,  however,  we  should  undertake 
to  carry  water  from  one  pole  of  the  earth  to  the  other 
pole  by  means  of  pipes,  we  could  accomplish  this  with 
the  least  resistance  by  employing  a  number  of  small  pipes 
of  small  diameter,  situated  like  meridians  of  the  earth, 
all  connecting  with  the  principal  mains  at  the  north  and 
south  poles.  The  greater  the  number  of  small  meridian 
pipes  the  less  the  resistance  to  the  flow  through  the 
large  mains. 

By  means  of  the  carbon  filament  in  an  exhausted 
globe  and  by  the  arrangement  of  the  multiple  circuit  the 
incandescent  system  of  electric  lighting  became  a  com- 
mercial success. 


CHAPTER  Y. 

FLOW   OF   ELECTRICITY   IN   THE    EAETH. 

IN  the  following  chapter  let  us  examine  the  passage 
of  electricity  through  the  earth ;  for  it  is  well  known 
that  it  was  discovered  in  the  early  days  of  telegraphy 
that  a  return  wire  between  Boston  and  New  York,  for 
instance,  cor  Id  be  dispensed  with,  and  that  the  earth 
could  be  used  instead  of  the  return  wire,  thus  halving 
the  injurious  resistance  of  the  circuit ;  for  it  was  found 
that  the  earth  did  not  oppose  any  appreciable  resist- 
ance compared  with  the  total  length  of  the  telegrapliic 
circuit. 

We  can  find  no  analogy  between  the  flow  of  steam, 
gas,  or  water  and  the  case  of  the  return  circuit 
through  the  earth.  In  the  case  of  steam,  gas,  and  water, 
and  of  all  fluids  forced  through  pipes  from  a  power- 
house, nothing  returns  to  the  power  house  if  we  should 
connect  the  pipes  to  the  ground ;  for  the  steam  would  be 
condensed,  the  air  pressure  lost,  and  the  water  would  soak 
into  the  ground.  In  the  case  of  electricity,  however, 
nothing  is  lost  by  connecting  the  wires  leading  from  the 
power  house  or  battery  to  the  ground.  Indeed,  in  cer- 
tain cases  a  great  deal  is  saved,  for  the  energy  of  the 
current  is  not  dissipated  into  heat  along  a  return  wire. 
"We  have  said  that  a  magnet  or  compass  needle  instant- 
ly points  to  a  wire  through  which  an  electrical  current 


53  WHAT  IS  ELECTRICITY? 

is  passing.  It  is  like  the  finger  of  a  mute  person  point- 
ing out  a  secret.  It  points  to  the  wire  if  it  is  moved 
along  the  wire  from  one  earth  plate  to  which  the  wire 
may  he  attached  to  the  power  house  or  battery,  and  from 
the  power  house  or  battery  to  the  earth  plate  at  the 
other  end  of  the  wire  circuit.  If  placed  on  the  earth 
between  these  earth  plates  and  sufficiently  far  from  the 
overhead  wire — on  the  ground,  for  instance,  beneath  an 
ordinary  telegraph  wire  strung  on  poles — the  compass 
or  magnet  is  quiescent  and  performs  its  normal  task  of 
pointing  to  the  poles  of  the  earth.  It  gives  no  evidence 
of  an  electric  current  in  the  ground  ;  the  electricity,  so 
to  speak,  seems  to  have  leaked  away  like  water.  Yet 
instruments  show  that  the  current  apparently  flows  out 
from  the  power  house  in  one  direction  to  the  ground 
and  returns  from  the  ground  to  the  power  house. 

If  we  should  take  a  minature  earth— a  globe  of  m .  1..1, 
for  instance — several  feet  in  diameter,  and  run  an  elec- 
tric current  to  what  may  be  called  the  north  pole  of  such 
a  globe  and  lead  it  away  from  the  south  pole,  we  shall 
find  that  the  current  apparently  spreads  out  from  the 
north  pole  and  converges,  so  to  speak,  to  the  south  pole. 
If  the  globe  were  20  feet  in  diameter  very  little  indica- 
tion of  a  current  would  be  obtained  around  the  equator 
of  such  a  globe.  Let  us  now  build  up  a  globe  made  of 
steam  or  water  pipes  all  connected  to  one  main  pipe  at 
the  north  pole,  and  again  at  the  south  pole.  We  can 
suppose  the  pipes  to  represent  the  divisions  of  an  orange. 
When  steam  or  water  leaves  the  main  pipe  and  is  divided 
in  its  flow  equally  among  the  pipes,  placed  similarly  to 
the  divisions  of  the  orange ;  the  amount  of  flow  through 
any  one  pipe  can  be  made  very  small,  although  the  flow 
through  the  main  pipe  leading  to  the  globe  is  very 
large.  If  we  should  connect  any  two  neighbouring 


FLOW  OF  ELECTRICITY  IN  THE  EARTH.         59 

pipes  along  the  -equator  so  that  water  or  steam  could 
now  from  one  to  the  other,  we  should  find  that  there 
would  be  no  flow,  for  the  pressure  at  the  two  ends  of 
such  a  connecting  pipe  is  the  same  ;  there  is  no  differ- 
ence of  pressure  to  force  the  steam  or  water  from  one 
pipe  to  the  other.  If,  however,  we  should  connect  one 
pipe  at  a  point  on  the  equator — in  Africa,  for  instance 
— with  another  pipe  at  a  point  corresponding  on  the 
globe  to  Isew  York,  there  would  be  a  flow  in  the  con- 
necting pipe,  for  there  would  be  a  difference  of  pressure. 
In  the  case  of  electricity,  a  telephone  will  determine 
whether  there  is  a  flow  from  one  portion  of  the  earth's 
surface  to  another  when  we  lead  an  electric  current  into 
the  earth  and  out  of  it.  Let  us  use  the  telephone  at 
first  merely  as  a  detector  of  an  electrical  flow,  just  as  we 
used  in  the  above  illustration  a  pipe  connecting  two  pipes 
in  order  to  determine  whether  there  is  any  possibility 
of  a  flow  of  water  between  them.  That  it  can  be  so 
used  we  can  easily  ascertain,  for  we  hear  a  click  in  the 
telephone  whenever  we  touch  the  two  wires  leading  to 
it  to  the  two  poles  of  an  ordinary  battery  such  as  is  used 
on  bell  wires  or  for  medical  purposes.  If  we  should 
hold  a  telephone  to  the  ear  and  connect  one  of  its  lead- 
ing wires  to  the  rail  of  an  ordinary  electric  road  and  the 
other  to  the  iron  posts  which  run  beside  the  track,  we 
should  hear  a  click  at  the  moment  of  making  the  contact 
with  the  iron  pole  if  there  is  a  leakage  of  electricity 
from  the  overhead  wire,  which  is  supported  by  the  iron 
pole  and  its  connections,  into  the  ground.  In  other 
words,  a  difference  of  electrical  level  would  be  shown 
between  the  iron  post  where  it  enters  the  ground  and 
the  rail.  If  now  we  should  make  a  globe  of  a  number 
of  copper  wires,  insulated  from  each  other  and  form- 
ing the  meridians  of  such  a  globe,  and  connect  all  these 


60  WHAT  IS  ELECTRICITY! 

great  circles  of  copper  wire  together  to  one  wire  at  the 
north  pole  and  to  another  wire  at  the  south  pole,  and  lead 
a  current  of  electricity  into  the  collection  of  meridian 
wires  at  the  north  pole  and  out  of  the  collection  at  the 
south  pole,  we  should  find  that  very  little  current  would 
go  through  any  one  wire  ;  and  if  we  should  connect  our 
telephone  wires  with  two  neighbouring  wires  anywhere 
along  the  equator  we  should  hear  no  click ;  there  is  no 
flow  of  electricity  between  points  of  the  same  pressure. 
If,  however,  we  connect  one  wire  of  the  telephone  at 
one  point  on  the  equator  and  the  other  wire  at  a  point 
on  the  wire  globe  corresponding  to  New  York,  we  should 
hear  a  click,  for  there  would  be  a  flow  between  these 
points. 

From  such  experiments  we  see  that  what  we  call  the 
electric  current  flows  out  in  all  directions  f  *vwi  the  point 
where  it  enters  the  earth,  and  appears  to  converge  again 
to  the  point  where  it  leaves  the  ground  to  enter  the  wire 
and  to  return  to  the  power  house  or  battery.  Perhaps 
the  best  illustration  of  the  manner  in  which  the  electric 
current  spreads  out  in  the  earth  is  afforded  by  a  method 
of  telegraphing  without  wires,  which  I  described  in  the 
Proceedings  of  the  American  Academy  of  Arts  and 
Sciences,  and  which  has  lately  been  repeated  by  Prof. 
Rubens  in  Berlin,  and  by  Mr.  Preece  of  the  London 
telegraphic  system.*  In  my  paper  I  remarked :  "  The 
theoretical  possibility  of  telegraphing  across  large  bodies 


*  My  original  researches  were  made  between  the  observatory  at 
Cambridge  and  the  city  of  Boston,  which  were  connected  by  a  time- 
signal  wire.  The  current  upon  this  wire  was  broken  by  a  clock  at 
regular  intervals.  I  found  that  I  could  hear  the  clock-beats  a  mile 
away  from  the  wire  by  connecting  a  telephone  to  a  wire  and  by 
grounding  the  ends  of  the  wire  500  or  600  feet  apart  and  parallel 
with  the  time  circuit. 


FLOW  OF  ELECTRICITY  Itf  THE  EARTH.         61 

of  water  is  evident  from  this  survey  which  I  have  under- 
taken. It  is  possible  to  telegraph  across  the  Atlantic 
Ocean  without  a  cable.  Powerful  dynamo-electric  ma- 
chines could  be  placed  at  some  point  in  Nova  Scotia, 
having  one  end  of  their  circuit  grounded  in  Florida, 
with  an  overhead  wire  between  these  points  of  great 
conductivity  and  carefully  insulated  from  the  earth  ex- 
cept at  the  two  grounds.  By  exploring  the  coast  of 
France  two  points  not  at  the  same  potential  could  be 
found,  and  by  means  of  a  telephone  of  low  resistance 
the  Morse  signals  sent  from  Nova  Scotia  to  Florida 
could  be  heard  in  France." 

What  we  have  said  in  regard  to  the  spreading  out 
of  the  electric  effect  or  current  in  the  earth  is  entirely 
applicable  to  the  case  of  the  human  body.  If  one  pole 
of  a  battery  or  other  source  of  electricity  is  applied  to 
the  middle  of  the  back  and  the  other  pole  to  the  middle 
of  the  breast,  the  electric  current  which  is  thus  led  into 
the  body  spreads  out  like  a  stream  of  water  through  an 
infinite  number  of  fine  holes  in  a  rose  jet ;  it  permeates 
every  muscle  in  a  greater  or  less  degree  between  the 
back  and  the  breast.  Its  flow  can  not  be  detected  in 
the  body  by  a  telephone,  but  a  delicate  galvanometer, 
which  is  the  electrician's  microscope,  will  show  its 
spreading.  Not  only  can  the  spreading  of  the  current 
be  detected  by  galvanometers,  but  this  action  can  also 
be  studied  by  chemical  analysis,  as  we  shall  see  when 
we  study  the  passage  of  electricity  through  fluids.  Be- 
fore going  further  in  the  subject  of  the  earth  circuit  we 
can  already  perceive  that  the  use  of  the  earth  for  a  re- 
turn is  not  always  desirable,  for  in  the  neighbourhood 
of  a  great  city  the  earth  becomes  filled,  so  to  speak,  with 
the  electrical  flow  from  the  common  use  of  the  earth  by 
the  telegraph  companies.  At  one  time,  as  I  have  shown 


62  WHAT  IS  ELECTRICITY? 

in  the  article  already  referred  to,  it  was  possible  to  ad- 
just one's  watch  by  connecting  a  telephone  to  the  water 
pipes  and  gas  pipes  in  almost  any  part  of  Boston  and 
Cambridge,  for  one  could  hear  the  clicks  of  the  obser- 
vatory clock  from  which  time  signals  were  sent.  The 
telephone  companies  no  longer,  however,  use  the  earth 
for  a  return  circuit  on  their  long-distance  lines,  and  em- 
ploy an  entire  metallic  circuit  of  copper  wire.  This 
circuit  obviates  the  earth  disturbances  due  to  the  spread- 
ing out  of  electric  circuits;  it  has  also  other  advan- 
tages, which  we  will  study  later.  It  is  interesting  to  ob- 
serve here  that  what  was  once  considered  a  notable 
practical  advantage  in  telegraphy  is  fast  losing  its  im- 
portance as  we  refine  upon  our  methods  of  transmitting 
intelligence  by  electricity.  We  p^ill  also  see  that  the 
earth  is  no  longer  used  by  the  electric  light  and  power 
companies.  We  shall  see  further  that  what  we  call  the 
steady  current  is  being  replaced  by  the  unsteady  current 
or  the  to-and-fro  current  for  the  electrical  transmission 
of  power  over  great  distances;  and,  still  stranger,  we  shall 
perceive  that  there  are  reasons  for  believing  that  there 
is  no  electric  current  or  flow  of  electric  energy  on  the 
wires  which  are  conveying  telegraphic  messages  or 
propelling  electric  cars ;  and  that  for  very  rapid  alter- 
nating currents  copper  is  really  a  poor  conductor,  and 
glass  an  excellent  one. 

There  are  several  terms  now  in  common  use  in  elec- 
trical science  which  serve  as  measures  of  value,  and  I 
shall  endeavour  to  give  a  popular  explanation  of  them. 
The  term  ampere  is  used  to  denote  the  strength  of  the 
electric  current ;  the  word  volt,  to  denote  the  unit  of 
electro-motive  force  or  electrical  pressure  on  the  circuit. 
The  current  may  be  said  to  flow  under  a  head,  which  is 
termed  the  voltage.  This  head  is  analogous  to  a  head 


FLOW  OP  ELECTRICITY  IN  THE  EARTH.    63 

of  water  which  forces  a  current  of  water  through  a  pipe. 
The  quantity  of  water  which  flows  through  the  pipe  in 
a  unit  of  time — say  a  second — is  a  measure  of  the  flow  of 
water ;  the  quantity  of  electricity  which  flows  in  a  sec- 
ond of  tune  is  a  measure  of  the  electrical  flow,  and  is 
called  an  ampere.  This  flow  meets  with  a  certain  elec- 
trical resistance,  which  is  termed  an  ohm.  The  flow  of 
water,  also,  through  a  pipe  meets  with  a  resistance  in 
the  friction  with  the  pipe.  These  terms — ampere,  volt, 
and  ohm — have  passed  into  daily  use.  They  perpetuate 
the  names  of  a  great  Frenchman,  a  great  Italian,  and  a 
great  German.  There  are  two  other  terms,  not  so  readi- 
ly comprehended  by  fluid  analogies :  The  farad,  named 
for  Faraday,  the  unit  of  electrical  capacity,  the  unit 
of  electrical  quantity  we  can  store  up  ;  and  the  henry, 
named  for  the  great  American  Joseph  Henry,  the  unit 
of  inductance  or  electrical  inertia — an  inertia  which 
manifests  itself  when  a  current  suddenly  rises  or  falls. 
These  two  terms — farad  and  henry — have  immense  im- 
portance in  the  subject  of  alternating  currents  of  elec- 
tricity. 


CHAPTEE  VI. 

THE   VOLTAIC   CELL. 

IT  is  interesting  to  reflect  that  the  study  of  elec- 
tricity received  its  greatest  primal  impulses  from  two 
nations  so  unlike  in  mental  characteristics — the  Anglo- 
Saxon  and  the  Italian.  To  Benjamin  Franklin  we  owe 
a  clearer  conception  of  the  phenomenon  of  lightning, 
and  to  Galvani  and  Volta  is  due  the  discovery  of  the 
electrical  battery.  In  later  times  we  are  indebted  to  the 
Anglo-Saxon  for  the  discovery  of  the  principle  under- 
lying the  action  of  the  dynamo  machine  and  the  tele- 
phone. The  story  of  the  discovery  of  the  electric 
battery  is  well  known,  but  we  will  repeat  it  for  the  sake 
of  pointing  out  modern  interpretations  of  the  mysterious 
action  which  puzzled  Galvani  and  Volta.  In  a  small 
cabinet  of  the  Jefferson  Physical  Laboratory  of  Harvard 
University  there  are  three  instruments  which  represent 
all  that  was  known  about  electricity  in  1830.  There  is 
a  Franklin  electrical  machine  of  great  size,  with  its  glass 
globe  and  its  rubbers,  and  its  ponderous  wheel  for  turn- 
ing the  globe  against  the  rubbers,  ordered  by  Benjamin 
Franklin  for  the  College  at  Cambridge ;  there  is  a  vol- 
taic battery  consisting  of  a  great  many  zinc  plates  and 
copper  plates  which  can  be  immersed  in  a  suitable  acid ; 
finally,  there  is  a  large  electro-magnet,  simply  a  horse- 
shoe-shaped piece  of  iron  wound  with  coarse  wire.  The 


THE  VOLTAIC  CELL.  65 

electrical  machine  and  the  battery  have  become  anti- 
quated ;  there  is  nothing  in  that  case  of  practical  use 
save  the  electro-magnet.  Moreover,  the  distinction  be- 
tween the  electrical  machine  and  the  battery  has  dis- 
appeared. We  shall  see  that  the  electrical  machine  is 
simply  a  battery,  and  that  when  Benjamin  Franklin 
rubbed  a  glass  rod  with  a  piece  of  catskin  he  was  dealing 
with  the  phenomenon  which  takes  place  in  any  voltaic 
cell. 

In  1791,  just  as  Benjamin  Franklin  was  passing  oS 
the  stage,  Galvani  made  the  observations  which  led  to 
the  voltaic  cell,  and  directed  philosophers'  contempla- 
tion from  the  clouds  to  the  phenomena  on  the  earth  and 
in  its  substances,  so  to  speak.  To  Galvani  we  owe  a  great 
debt,  greater  far,  I  believe,  than  we  owe  to  Franklin, 
for  Galvani  opened  a  vast  field  of  inquiry  which  led  to 
the  discovery  of  electro-magnetism,  and  gave  Faraday 
the  means  of  discovering  induction.  Before  Galvani's 
time  men  were  lost  in  philosophical  speculations  in  re- 
gard to  subtle  fluids.  After  his  experiments  their 
thoughts  were  directed  to  the  conditions  of  matter  im- 
mediately about  them.  Benjamin  Franklin  brought 
electricity  down  to  earth  from  the  clouds,  while  Gal- 
vani's experiments  brought  men's  minds  down  from 
the  heights  where  they  were  lost,  having  no  tangible 
transformations  to  study.  His  own  account  of  the  be- 
ginning of  his  experiments  is  extremely  interesting,  for 
it  shows  that  he  was  not  anxious  to  make  it  appear  that 
he  was  the  first  to  notice  the  strange  phenomena  which 
proved  to  have  such  far-reaching  results.  He  says : 

"  This  is  the  way  the  thing  happened.  I  dissected  a 
frog  and  prepared  it  as  is  shown  in  Fig.  6,  and  laid  it 
on  a  table  upon  which  stood  an  electrical  machine 
(Fig.  6),  far  from  the  prime  conductor  and  not  in  a 


66  WHAT  IS  ELECTRICITY! 

straight  line  with  it.  When  one  of  the  servants,  who 
was  at  hand,  touched  with  the  point  of  the  dissecting 
knife  the  inner  lumbar  nerve  (D  D)  of  the  frog,  all  the 
muscles  of  the  thighs  appeared  to  contract  as  if  under 
the  influence  of  powerful  cramps.  The  assistant  thought 
that  the  phenomenon  occurred  when  a  spark  passed  be- 
tween the  conductors  of  the  electrical  machine.  Aston- 
ished by  this  new  phenomenon,  he  turned  to  me,  I  being 
occupied  in  other  matters  and  absorbed  in  thought. 
Thereupon  I  was  inflamed  by  an  incredible  haste  and 
desire  to  prove  the  same,  and  bring  the  hidden  mystery 
to  light."* 

Here,  with  extreme  candidness  and  generosity,  he 
gives  full  credit  to  his  assistants  for  the  accidental  dis- 
covery. He  traced  the  cause  of  the  strange  convulsions 
to  the  working  of  the  electrical  machine,  and  found  that 
discharges  of  lightning  could  also  produce  the  move- 
ments of  the  muscles  of  the  frog's  legs.  He  continues 
thus: 

"  After  I  had  investigated  the  effects  of  atmospheric 
electricity  my  heart  burned  with  desire  to  test  the  power 
of  the  daily  quiet  charge  of  electricity  in  the  atmos- 
phere." He  had  noticed  that  the  prepared  frog's  legs, 
bound  up  with  brass  hooks  on  an  iron  railing,  showed  the 
same  contractions  not  only  in  the  case  of  thunderstorms, 
but  also  under  a  clear  sky.  He  was  thereupon  led  to 
study  the  effect  of  touching  the  nerves  with  different 
metals  and  with  non-conductors,  such  as  glass  and  wax. 
He  found  that  when  the  circuit  between  the  nerves  was 
made  by  two  different  metals  powerful  contractions  en- 
sued. 


*De  Bononiensi  Scientiarum  et  Artium  Institute  atque  Acad- 
emia  Commentarii,  tomus  vii. 


68  WHAT  IS  ELECTRICITY? 

In  Fig.  6  we  see  a  representation  of  the  various  ex- 
periments which  he  tried.  The  twitching  of  the  frog's 
legs  served  him  for  a  galvanometer — a  sensitive  indica- 
tor of  the  electrical  current  which  was  excited  in  the 
circuit  of  the  metals  and  the  muscles  and  nerves  of  the 
frog.  A  new  instrument  in  physical  science  often  opens 
a  great  field  of  discovery.  The  frog's  legs  in  the  hands 
of  Galvani  and  his  co-workers  proved  to  be  such  an  in- 
strument. 

It  was  not  a  difficult  step  to  take  from  the  stand- 
point of  Galvani  to  that  of  Yolta.  The  instrument  was 
at  hand  and  the  phenomenon  had  been  observed.  Gal- 
vani attributed  the  action  to  the  vital  electricity  of  the 
nerves  and  the  muscles  of  the  frog,  while  Yolta  attrib- 
uted the  action  entirely  to  the  contact  action  at  the 
junction  between  the  two  metals.  The  controversy  as 
to  the  cause  and  seat  of  the  electro -motive  force  between 
two  metals  in  an  ordinary  battery  is  still  a  matter  of  dis- 
pute, and  we  are  little  wiser  than  philosophers  were  in 
the  days  of  Galvani  and  Volta.  The  labours  of  these 
two  men,  however,  opened,  as  we  have  said,  a  great 
field  in  electricity.  Men  began  to  study  the  manifold 
phenomena  of  the  electric  current  produced  by  batter- 
ies, and  the  next  great  step  was  made  by  Oersted,  who 
discovered  the  principle  of  a  new  instrument,  the  gal- 
vanometer, the  indications  of  which  have  led  the  way  to 
the  great  practical  employment  of  electricity. 

Galvani,  we  have  said,  attributed  the  source  of  the 
electrical  current  which  convulsed  the  frog's  leg  to  the 
animal  electricity  of  the  frog.  Yolta,  however,  soon 
showed  that  an  electrical  effect  could  be  obtained  with- 
out the  use  of  the  frog's  leg  by  connecting  a  piece  of 
zinc  to  a  piece  of  copper,  and  he  was  led  to  the  inven- 
tion of  the  voltaic  pile,  which  in  its  first  form  consisted 


THE  VOLTAIC  CELL.  69 

merely  of  alternate  discs  of  copper  and  zinc  separated 
by  pieces  of  blotting  paper  moistened  with  salt  and  water. 
Yolta  attributed  the  effect  observed  by  Galvani  to  the 
contact  between  the  two  metals  which  were  employed  to 
touch  the  frog's  legs.  In  an  ordinary  battery  the  plates 
forming  the  battery  are  not  in  contact,  but  are  separated 
by  a  layer  of  liquid,  which  acts  chemically  upon  one  of 
the  plates.  The  question  of  the  seat  of  the  electro-mo- 
tive force  in  a  voltaic  cell  is  as  much  a  mystery  to-day 
as  it  was  in  the  times  of  Galvani  and  Yolta,  but  we  are 
beginning  to  see  that  our  best  hope  of  solving  the  mys- 
tery consists  in  studying  the  transformations  of  energy 
in  the  battery,  and  in  measuring  the  heat  developed 
under  different  conditions.  The  chemical  action  is  an 
evidence  of  an  increased  molecular  activity,  so  to  speak, 
and  in  general  terms  we  can  conclude  that  whenever  in 
a  circuit  we  have  a  difference  of  molecular  activity,  at 
two  points  in  the  circuit  an  electrical  circuit  results. 
The  phenomena  which  takes  place  in  a  battery  consist- 
ing of  two  metals  with  a  liquid  between  the  two  metals 
appear  to  be  far  more  complicated  than  those  which  are 
manifested  along  the  wire  connecting  the  metals  or  along 
the  outer  circuit.  The  liquid  is  broken  up.  If  it  is 
acidulated  water,  oxygen  is  given  off  at  the  positive  pole 
and  hydrogen  at  the  negative. 

It  can  be  said,  in  general,  that  an  electrical  current  is 
generated  whenever  two  dissimilar  metals  connected  by 
a  wire  are  immersed  in  a  liquid  which  is  capable  of  con- 
ducting electricity.  For  instance,  if  the  handles  of  a 
silver  spoon  and  an  iron  spoon  are  connected  by  a 
copper  wire,  and  the  bowls  of  the  spoon  are  immersed  in 
a  tumbler  of  salt  and  water,  an  electric  current  passes 
from  the  silver  to  the  iron  along  the  copper  wire,  and 
in  the  water  from  the  iron  spoon  to  the  silver  spoon. 


70  WHAT  IS  ELECTRICITY? 

Such  a  battery  would  be  sufficient  to  send  a  signal  under 
the  Atlantic  from  America  to  England,  but  it  would 
not  be  powerful  enough  for  commercial  use.  Before 
the  iron  spoon  is  connected  with  the  silver  spoon  by  a 
wire,  very  delicate  instruments  will  show  a  negative 
charge  of  electricity  upon  the  iron  spoon,  and  a  positive 
charge  upon  the  silver  spoon.  By  the  use  of  a  great 
number  of  zinc  and  copper  plates  separated  by  paper 
moistened  with  salt  and  water,  Yolta  was  able  to  show 
that  a  body  charged  with  positive  electricity  was  repelled 
by  the  terminal  connected  to  the  copper  plates,  and  at- 
tracted by  that  connected  to  the  zinc  plates.  Indeed, 
by  greatly  increasing  the  number  of  tumblers  contain- 
ing plates  of  copper  and  zinc  immersed  in  salt  and  water, 
and  connecting  each  copper  plate  to  each  zinc  plate  by 
a  wire,  one  can  obtain  electrical  sparks  when  the  final 
copper  plate  is  brought  near  to  the  first  zinc  plate. 

Since  any  two  metals  immersed  in  a  suitable  con- 
ducting liquid  constitutes  a  voltaic  cell  or  battery,  it  will 
be  readily  seen  that  the  number  of  forms  of  such  bat- 
teries is  very  great.  The  Leclanche  cell,  which  is  so 
commonly  employed  at  present  in  houses  to  supply  the 
electric  current  for  bells,  consists  mainly  of  a  rod  of 
zinc  and  a  rod  of  carbon,  both  immersed  in  a  solution 
of  sal  ammoniac.  When  Prof.  Tyndall  came  to  America 
to  deliver  lectures  on  physical  science,  he  brought  with 
him  a  hundred  Grove  cells,  which  consist  of  platinum 
plates  immersed  in  strong  nitric  acid  which  is  contained 
in  a  porous  cup ;  this  cup  is  placed  in  another  receptacle 
filled  with  sulphuric  acid  and  water,  in  which  there  is  a 
zinc  plate.  The  electrical  current  flows  from  the  plat- 
inum to  the  zinc  outside  the  cell,  and  from  the  zinc  to 
the  platinum  inside  the  cell.  The  cells  are  joined,  as 
we  have  said,  in  series,  zincs  to  platinums,  and  finally 


THE  VOLTAIC  CELL.  71 

the  last  zinc  is  joined  to  the  first  platinum.  By  means 
of  fifty  such  cells  Prof.  Tyndall  showed  the  electric 
light,  and  illustrated  the  subject  of  optics  by  its  aid. 
The  labour  of  preparing  such  a  battery  for  each  lecture 
was  very  great,  the  nitrous  acid  fumes  were  dan- 
gerous, and  the  battery  would  not  furnish  a  current 
sufficient  for  the  production  of  light  equal  to  an  ordi- 
nary street  electric  lamp  for  more  than  an  hour.  The 
lecturer  on  physics  to-day  finds  a  source  of  electricity 
on  hand  similar  in  abundance  and  ease  of  application  to 
the  supply  of  gas  or  water.  This  supply  of  electricity, 
however,  is  obtained  not  from  batteries  but  from 
dynamo  machines.  We  shall  describe  the  dynamo  in  a 
subsequent  chapter,  and  point  out  what  a  revolution  it 
has  accomplished  in  the  practical  uses  of  electricity.  The 
voltaic  cell,  however,  can  never  lose  its  scientific  im- 
portance, although  it  has  ceased  to  be  looked  upon  as  a 
commercial  source  of  electricity  for  lighting  purposes 
or  for  the  transmission  of  power.  Why  two  different 
metals  immersed  in  a  conducting  liquid  produce  a  cur- 
rent of  electricity  is  not  well  understood.  This  question 
was  the  cause  of  a  memorable  discussion  between  Yolta 
and  Galvani,  as  we  have  seen,  and  within  a  few  years 
the  British  Association  appointed  a  committee  to  col- 
lect and  classify  the  investigations  on  the  seat  of  the 
electro-motive  force  in  a  voltaic  cell. 

I  have  said  that  the  battery  is  still  of  great  scientific 
importance.  We  are  brought  face  to  face  in  its  action 
with  the  great  problem  of  the  connection  of  electrical 
action  and  molecular  movement ;  indeed,  with  the  funda- 
mental principles  of  chemical  action.  Modern  chemistry 
is  rapidly  becoming  a  study  of  motion,  and  this  can  also 
be  said  of  electricity.  We  have,  then,  a  common  ground 
of  attack  in  endeavouring  to  penetrate  into  the  mysteries 


72  WHAT  IS  ELECTRICITY  t 

of  chemical  action  and  electrical  action.  When  we  touch 
the  lower  surface  of  our  tongue  with  the  wire  from  one 
plate  or  pole  of  a  battery,  and  the  upper  surface  with  the 
wire  leading  to  the  other  pole  of  the  battery,  we  can 
taste,  so  to  speak,  the  electric  current.  We  are  conscious 
of  a  peculiar  tingling  sensation  hi  the  tongue.  If  we 
lead  the  wire  into  acidulated  water,  we  find  that  the  water 
is  broken  up  into  its  constituents — bubbles  of  hydrogen 
gas  are  given  off  at  one  end  of  the  wire  immersed  in  the 
water,  and  bubbles  of  oxygen  at  the  end  of  the  other  wire. 
This  action  is  called  electrolysis,  and  we  taste  it,  so  to 
speak,  when  we  touch  our  tongue,  with  its  tissues  rich  in 
fluids,  to  the  wire  terminals.  Instead  of  the  sensation 
of  taste,  it  is  rather  a  slight  electric  spark  which  we  feel, 
which  arises  from  the  electro-motive  force  which  pro- 
duces electrolysis.  The  action  of  electrolysis  is  immedi- 
ately connected  with  molecular  motion.  The  electrical 
action  has  rent  asunder  the  bonds  that  connected  the 
hydrogen  molecule  to  the  oxygen  molecule — the  bond 
that  made  up  the  particles  of  water — and  these  hydrogen 
and  oxygen  molecules  are  now  free  to  vibrate  independ- 
ently of  each  other,  and  to  enter  into  new  combinations. 
We  shall  find  that  their  mere  adherence  to  the  ends  of 
the  wires  dipping  into  the  acidulated  water  is  sufficient 
to  produce  an  electric  current ;  for  if  we  remove  the 
battery  which  has  broken  up  the  water  into  its  con- 
stituents, and  connect  together  the  ends  of  the  wire 
which  were  connected  to  the  poles  of  the  battery,  we  shall 
obtain  again  an  electrical  current.  Furthermore,  we 
find  that  by  means  of  electrical  action  we  can  transport 
a  substance  through  the  tissues  of  the  human  body — for 
instance,  from  the  back  to  the  breast  of  a  man. 

The  electrical  current,  therefore,  flowing  through  a 
conducting  fluid,  exerts  a  marked  effect  upon  the  mole- 


THE  VOLTAIC  CELL.  f3 

cules  of  the  fluid.  In  this  respect  its  behaviour  in  pass- 
ing through  a  fluid  differs  greatly  from  that  which  it 
manifests  in  a  wire  or  metal  conductor.  No  molecular 
effect  has  yet  been  observed  in  wires  through  which  cur- 
rents of  electricity  have  passed,  save  the  effect  due  to 
the  heating  of  the  wires.  Since  the  earth  is  composed 
of  both  solid  and  liquid  matter,  we  should  expect  to  find 
some  evidence  of  chemical  action  set  up  by  the  spreading 
of  the  electric  current.  This  evidence  is  very  strong  in 
the  case  of  electric  railroads.  The  current  used  by  such 
roads  returns  to  the  power  house  by  the  rails  of  the  road, 
by  conductors  connected  with  these  rails,  and  by  the 
earth.  The  current  spreads  out  in  the  earth,  and  seeks 
the  shortest  passage  back  to  the  power  house.  It  has 
been  found  that  the  gas  and  water  pipes  of  a  city  hi 
which  an  electric  road  is  situated  exhibit  a  chemical  ac- 
tion due  to  the  electrical  current  of  the  railroad.  This 
action  is  similar  to  that  we  observe  on  the  ends  of  wires 
which  lead  an  electrical  current  to  and  away  from  a 
conducting  liquid.  For  instance,  bubbles  of  oxygen 
adhere  to  the  end  of  the  wire  by  which  the  current 
enters  a  tumbler  of  salt  and  water,  and  bubbles  of 
hydrogen  to  the  end  by  which  the  current  leaves  the 
tumbler.  This  action  is  called  electrolytic  polarization. 
It  was  found  in  Boston  lately  that  the  lead  pipes  in  cer- 
tain localities  were  filled  with  minute  piuholes  where 
this  electrolytic  action  had  eaten  away  the  lead.  This 
action  has  been  lessened  by  connecting  the  positive  pole 
of  the  source  of  electricity — the  dynamo — with  the  rails, 
and  the  underground  pipes  and  the  negative  pole  with 
the  overhead  wire,  so  that  the  current  of  electricity 
should  flow  from  the  underground  pipes,  the  earth,  and 
the  rails  through  the  wheels  of  each  car  to  the  trolley, 
and  back  to  the  power  station  along  the  overhead  wire. 


74  WHAT  IS  ELECTRICITY! 

In  this  way  the  electrolytic  action  is  exerted  less  strongly 
on  the  underground  pipes. 

The  most  striking  example  of  this  action  of  the  cur- 
rent in  passing  through  a  liquid  is  in  the  case  of  what  is 
called  the  storage  battery.  If,  instead  of  leading  a  cur- 
rent of  electricity  into  a  tumbler  of  water  and  out  of  it 
by  means  of  copper  wires,  we  should  connect  the  lead- 
ing-in  wire  to  a  strip  of  lead,  and  the  outgoing  wire  also 
to  a  strip  of  lead — the  lead  strips  being  immersed  in 
the  water  in  the  tumbler — we  should  have  an  elementary 
modern  lead  storage  cell.  Bubbles  of  hydrogen  adhere 
to  one  lead  strip,  and  bubbles  of  oxygen  to  the  other, 
when  a  sufficiently  strong  electric  current  is  passed 
through  the  water  in  the  tumbler.  These  gases  arise 
from  the  decomposition  of  the  water  by  the  electric  cur- 
rent, and  this  decomposition,  we  have  said,  is  called 
electrolysis.  The  amount  of  oxygen  and  hydrogen  ad- 
hering to  the  lead  strips  can  be  enormously  increased  by 
covering  the  lead  strips  with  oxide  of  lead.  The  oxygen 
given  off  on  one  lead  strip  is  held  there  in  great  quantity 
by  the  conversion  of  the  oxide  of  lead  into  a  higher  ox- 
ide, that  is,  an  oxide  with  a  larger  amount  of  oxygen — 
the  peroxide  of  lead — while  the  hydrogen  gas  lowers  the 
oxide  of  lead  on  the  other  lead  strip  to  a  finely  divided 
porous  layer  of  hydrogen  ated  lead.  When  such  lead 
strips,  after  having  been  charged  by  an  electric  current, 
are  connected  by  an  external  wire,  a  strong  current 
rushes  from  the  oxygenated  lead  strip  to  the  hydrogen- 
ated  strip.  By  increasing  the  surface  of  such  strips  we 
can  charge  such  a  storage  battery  to  an  enormous  amount. 
If  Prof.  Tyndall  could  have  had  a  storage  battery  of  fifty 
cells  instead  of  fifty  pint  Grove  cells,  the  charging  of 
such  a  battery  would  have  sufficed  for  an  entire  course 
of  lectures,  and  the  labour  of  preparation  would  have 


THE  VOLTAIC  CELL.  75 

been  greatly  diminished.  It  will  be  observed  that  the 
term  storage  battery  is  a  misleading  one,  if  we  under- 
stand by  the  term  storage  a  storage  of  electricity.  What 
is  really  stored  is  chemical  action  due  to  the  electrical 
current. 

One  of  the  chief  practical  objections  to  the  employ- 
ment of  the  lead  storage  battery  is  its  weight.  A  lead 
battery  sufficiently  powerful  to  propel  a  bicycle  or  tri- 
cycle, developing  one  quarter  of  a  horse  power,  would 
weigh  at  least  300  pounds,  and  it  would  not  furnish 
power  for  more  than  four  hours  before  it  would  need  to 
be  recharged.  Prof.  Langley  informs  me  that  he  has 
constructed  a  little  steam  engine  which  weighs  less  than 
two  pounds  and  which  develops  one  quarter  of  a  horse 
power.  Some  of  the  recent  forms  of  petroleum  motors 
for  horseless  carriages  develop  at  least  two  horse  power 
and  weigh  only  fifteen  pounds.  It  will  be  seen,  there- 
fore, that  the  storage  battery  in  its  present  best  form, 
which  is  the  lead  form,  is  not  of  much  practical  im- 
portance in  the  subject  of  the  transmission  of  power ; 
it  is,  however,  of  the  greatest  theoretical  interest. 

With  five  thousand  little  lead  cells  charged  by  a 
dynamo  machine  sparks  can  be  obtained,  and  a  severe 
electric  shock  is  felt  when  the  positive  and  negative 
terminals  of  the  battery  are  seized  by  the  hands.  The 
battery  imitates  in  all  respects  the  action  of  Frank- 
lin's electrical  machine  ;  and  it  is  interesting  to  observe 
that  at  first  the  celebrated  experiment  of  Galvani  seemed 
to  lead  us  away  from  the  study  of  f rictional  electricity, 
which  had  exclusively  filled  philosophers'  minds  before 
the  date  of  Gralvani's  experiments.  Our  increase  of 
knowledge,  however,  of  galvanism  has  conducted  us 
back  to  the  field  in  which  Benjamin  Franklin  worked, 
giving  us  a  little  more  light  upon  the  subject  of  what  is 


76  WHAT  IS  ELECTRICITY! 

called  f  Fictional  electricity.  At  first  sight  nothing  seems 
more  remote  from  the  phenomena  produced  by  rubbing 
a  cat's  fur,  or  the  phenomenon  of  lightning,  than  the 
action  of  the  battery  which  rings  bells,  decomposes 
liquids,  and  produces  the  mysterious  magnetic  effect  in 
the  neighbourhood  of  wires  which  we  have  noted.  We 
have  now  good  reasons  for  believing  that  when  we  stroke 
the  fur  of  a  cat  the  mechanical  action  breaks  up  the 
arrangement  of  the  molecules  of  the  hair  and  of  the  sur- 
face of  the  hand,  just  as  the  electro-motive  force  of  a 
battery  breaks  up  the  arrangement  of  molecules  in  a 
conducting  fluid  and  produces  an  electric  charge  on  the 
two  metals  of  the  battery.  When  a  lady  produces  a 
Bpark  by  walking  across  a  carpeted  floor,  the  molecules 
of  her  silk  skirts  and  dry  garments  are  rudely  dis- 
arranged and  an  electric  charge  results.  This  charge 
does  not  result  from  any  peculiar  electricity  of  the  body. 
It  is  not  what  is  termed  animal  electricity.  It  results 
merely  from  the  rubbing  of  the  clothes  or  of  the  dry 
slippers  upon  the  carpet,  and  it  can  be  produced  by  men 
as  well  as  women,  being  merely  a  question  of  the  proper 
clothing  necessary  for  its  production. 

In  our  furnace-heated  houses  in  winter  the  phenom- 
ena of  electrical  charges  produced  by  friction  is  of 
common  occurrence.  A  pair  of  silk  undergarments 
suddenly  withdrawn  from  a  pair  of  trousers  diverge 
under  their  electrical  charge.  A  sheet  of  paper  briskly 
rubbed  adheres  to  objects  presented  to  it.  The  sheet  of 
paper,  together  with  the  object  which  is  brought  near  it, 
really  constitute  a  battery — the  sheet  being  one  pole 
and  the  neighbouring  object  being  the  other  pole,  while 
the  air  between  takes  the  place  of  the  fluid  in  the  ordi- 
nary battery.  The  Franklin  electrical  machine  can  be 
considered  a  battery,  and  can  be  made  to  produce  all  the 


THE  VOLTAIC  CELL.  77 

effects  produced  by  an  ordinary  voltaic  cell.  We  shall 
see,  when  we  consider  the  dynamo,  that  the  latter  also 
can  produce  all  the  effects  due  to  batteries,  and  it  can 
be  made  to  give  sparks  five  feet  long  which  are  identi- 
cal with  discharges  of  lightning. 

In  general,  a  change  of  molecular  aggregation  pro- 
duces a  manifestation  of  electricity,  and  when  we  reflect 
upon  this  fact  we  see  the  immense  importance  to  the 
chemist  of  the  study  of  electricity.  If  we  should  sud- 
denly bend  a  ring  of  metal  wire  we  can  produce  a  current 
of  electricity  in  the  wire.  Moreover,  if  we  hold  it  over 
a  lamp  and  heat  it  at  one  point  we  can  also  produce  a 
current  in  it.  In  these  cases  also  the  current  is  produced 
by  the  change  hi  molecular  arrangements  in  the  wire 
ring.  "We  are  apt  to  hastily  conclude,  from  observa- 
tions upon  the  electricity  produced  by  differences 
in  molecular  activity,  that  electricity  is  a  motion  or 
vibration  of  molecules.  As  we  continue,  however,  our 
study  of  what  electricity  is,  we  perceive  that  there  are 
other  and  more  significant 
ways  of  producing  electrici- 
ty than  by  chemical  action, 
or  by  any  operation  wliich 
breaks  up  and  changes  the 
arrangement  of  molecules. 

A  voltaic  cell  is  not  our 
only  means  of  obtaining  a 

current  of  electricity  without  the  use  of  a  dynamo. 
One  of  the  most  valuable  means  for  an  experimenter 
consists  iii  the  employment  of  heat.  If  we  join  any 
two  metals — for  instance,  iron  and  copper — and  heat 
one  junction — for  instance,  A,  Fig.  7 — and  cool  the 
other,  I>,  an  electric  current  results  which  flows  from 
the  hot  junction  through  the  copper  wire  to  the  cold 


78  WHAT  IS  ELECTRICITY  I 

junction.  The  new  alloy  termed  constantan,  with  cop- 
per, gives  a  very  sensitive  combination.  These  thermo- 
electric junctions  can  be  used  as  very  delicate  thermome- 
ters. One  can  easily  determine  the  one  hundredth  of  a 
degree  Centigrade  by  means  of  them.  To  detect  the 
current,  it  is  merely  necessary  to  have  a  delicate  galva- 
nometer. Prof.  Tyndall,  in  his  remarkable  treatise  on 
Heat  as  a  Mode  of  Motion,  devotes  much  space  to  the 
description  of  his  thermopile  and  the  galvanometer  he 
employed,  for  this  apparatus  served  to  illustrate  through- 
out his  treatise  the  various  manifestations  of  heat  due  to 
motion.  He  used  it  instead  of  a  thermometer.  Since 
the  date  of  the  publication  of  Prof.  Tyudall's  treatise, 
which  gave  to  the  world  in  a  popular  form  the  great 
generalization  of  the  conservation  of  energy,  this  form 
of  electrical  thermometer  has  been  much  simplified  and 
made  more  sensitive.  Instead  of  the  needle  galvanom- 
eter employed  by  Prof.  Tyndall,  we  now  have  the  mir- 
ror galvanometer,  and  we  are  able  to  detect  electrical 
currents  which  his  instrument  would  not  respond  to. 

Thermal  currents  always  arise  when  there  is  a  differ- 
ence of  temperature  between  any  two  points  in  an  elec- 
tric circuit.  It  is  not  necessary  to  employ  the  junction 
of  the  different  metals  to  produce  these  currents.  If  a 
piece  of  copper,  for  instance,  is  heated  at  one  place  and 
cooled  at  another,  a  thermo-electric  current  results.  If 
a  knot  is  tied  in  it  and  it  is  heated  on  one  side  of  the 
knot,  a  current  results.  In  other  words,  any  change  in 
the  molecular  aggregation  of  the  metal  at  the  points 
will  produce  a  current  of  electricity.  An  attempt  has 
been  made  to  apply  this  principle  to  practical  use  ;  for 
instance,  a  furnace  has  been  constructed  with  thermal 
junctions  set  in  its  walls,  with  the  other  set  of  junctions 
outside.  It  is  possible,  by  having  a  large  number  of 


THE  VOLTAIC  CELL.  79 

junctions  in  the  pot  of  a  furnace,  to  produce  an  electric 
light.  The  heat  of  an  ordinary  house  furnace  is  suffi- 
cient to  afford  a  supply  of  electricity  for  the  house  bells 
and  for  running  a  sewing-machine,  and,  indeed,  for  a 
certain  amount  of  electric  lighting;  but  no  practical 
way  has  been  devised  of  preserving  the  junctions  from 
injury  due  to  expansions  and  contractions.  The  energy 
in  the  coal  we  use  in  ordinary  furnaces  is  amply  suffi- 
cient both  to  heat  and  light  our  houses  if  it  could  be 
economically  transformed.  It  has  been  proposed  by 
various  investigators  to  employ  the  electro -motive  force 
developed  between  iron  and  carbon  which  are  im- 
mersed in  a  hot  alkali.  The  carbon  wastes  away  or  is 
consumed  under  the  action  of  a  current  of  air  which  is 
forced  through  the  melted  alkali.  If  we  could  break 
up  the  carbon  into  its  constituents,  or  ions,  as  we 
can  water,  we  could  produce  electricity  directly  from 
coal.  The  use  of  the  thermal  junction  seems  at  present 
the  only  way  wrorthy  of  consideration  by  means  of  which 
we  can  produce  electricity  direct  from  coal  without  the 
use  of  a  steam  engine.  The  electro-motive  force  pro- 
duced by  heating  the  junction  of  two  metals,  however, 
is  very  small  even  with  the  best  combination — bismuth 
and  antimony — far  less  than  with  the  employment  of 
copper  and  zinc  in  the  ordinary  voltaic  cell. 

Both  with  the  employment  of  two  different  metals 
and  in  the  case  of  an  electrolyte,  outside  the  evidence 
of  the  increased  molecular  activity  at  the  surface  of  the 
metals  and  an  increase  of  heat  throughout  the  liquid  of 
the  cell,  there  is  no  evidence  of  any  flow  in  one  direc- 
tion or  the  other,  or  of  any  commotion  in  the  liquid. 
At  a  sufficient  distance  from  the  plate  of  the  battery 
the  liquid  is  in  repose.  A  beam  of  light  sent  through 
it  is  not  absorbed  by  the  liquid  more  or  less  when  the 


80  WHAT  IS  ELECTRICITY? 

current  flows.  A  photograph  taken  by  means  of  light 
which  has  passed  through  a  layer  of  liquid  traversed  by 
an  electric  current  differs  in  no  respect  from  one  taken 
through  an  equal  layer  of  the  liquid  not  traversed  by  a 
current.  No  effect  of  strain  can  be  observed  in  the 
liquid.  Faraday  examined  this  question  carefully  by 
means  of  the  polarization  of  light.  There  are  certain 
substances  which  confine  the  vibrations  of  light  in  one 
plane.  We  can  think  of  such  substances  as  similar  in 
internal  arrangement  to  a  Venetian  blind  with  its  hori- 
zontal slats.  If  we  should  attempt  to  thrust  a  hand 
moving  to  and  fro  through  the  blind,  it  would  be  possi- 
ble to  do  so  only  when  the  hand  is  moving  to  and  fro 
horizontally.  Now  the  vibrations  of  a  ray  of  light  are 
in  all  directions,  so  that  a  section  of  the  ray  would  be  a 
star-shaped  figure.  Only  those  vibrations  which  are  in 
the  horizontal  plane  would  therefore  pass  through  the 
blind.  When  this  light  emerges  from  such  an  arrange- 
ment of  matter,  therefore,  it  is  said  to  be  polarized,  and 
it  can  be  tested  by  another  substance  exactly  similar  to 
that  which  polarizes  the  ray ;  for  if  the  analyzer  is 
turned  so  that  its  molecular  arrangement,  in  regard,  for 
instance,  to  a  horizontal  plane,  is  the  same  as  the  polar- 
izer— that  is,  if  two  Venetian  blinds  are  hung  vertically 
one  behind  the  other — the  hand  moving  horizontally 
can  be  made  to  pass  through  both.  If  one  bh'nd,  how- 
ever, is  not  vertical,  and  the  slats  are  at  right  angles 
with  the  other,  the  movements  of  the  hand  will  be  in- 
tercepted. Two  Mcol  prisms  form  such  an  arrange- 
ment. Although  they  are  perfectly  limpid,  like  two 
pieces  of  glass,  yet  when  we  look  through  both  and 
turn  one  we  shall  find  that  all  the  light  is  cut  off  in  a 
certain  position  of  the  prism  next  the  eye,  which  is 
called  the  analyzer.  When  a  piece  of  glass  submitted 


THE  VOLTAIC  CELL.  81 

to  a  strain  is  placed  between  the  two  Nicols,  we  find  that 
we  are  obliged  to  turn  the  analyzer  through  an  angle 
to  obtain  extinction  of  the  light,  which  is  different  from 
the  angle  of  extinction  when  the  glass  is  not  under  strain. 
Polarized  light,  therefore,  enables  us  to  detect  a  strain. 
Faraday  could  not,  by  an  arrangement  similar  to  that 
we  have  described,  detect  any  strain  either  in  the  line 
between  the  two  poles  or  in  a  direction  at  right  angles 
to  this. 

Besides  the  chemical  effects  noticed  in  the  voltaic 
cell,  there  are  other  effects  which  are  different  from 
those  noticed  on  the  outer  circuit.  For  instance,  the 
electrical  resistance  through  the  liquid  of  the  cell  di- 
minishes when  the  cell  is  heated,  whereas  the  resistance 
in  the  metallic  circuit  increases  when  the  conductor  is 
heated.  Moreover,  there  is  no  polarization  effect  ob- 
servable in  a  conductor  carrying  a  current — that  is,  no 
current  can  be  obtained  from  two  portions  of  a  wire 
through  which  an  electric  current  has  passed.  Yet  the 
same  portions,  on  being  placed  in  a  liquid  capable  of 
being  broken  up  by  the  electrical  current,  and  after 
being  traversed  by  a  current,  exhibit  what  is  termed 
polarization ;  they  give  an  electrical  current  which  is 
opposed  to  the  original  current.  In  discussing  these 
two  points  of  marked  difference  in  behavior  between 
metallic  conductors  and  electrolytes,  or  liquids  which 
conduct  and  are  decomposed,  Prof.  J.  J.  Thomson 
remarks  that  these  differences  are  not  so  marked  as  to 
constitute  real  differences.  He  points  out  from  the 
molecular  theory  that  if  any  polarization  took  place  in 
the  metallic  conductor  it  would  disappear  so  quickly 
that  it  would  elude  observation.  In  the  case  of  alloys, 
Prof.  Austen  Roberts  was  not  able  to  detect  any  of  the 
constituents  of  the  alloy  at  either  electrode.  Prof. 


82  WHAT  IS  ELECTRICITY? 

Thomson  again  shows  that  the  effect,  if  any,  would  dis- 
appear with  great  rapidity.  With  regard  to  the  differ- 
ence between  conduction  in  metals  and  in  electrolytes, 
he  states  that  he  has  discovered  that  in  an  amalgam 
containing  about  30  per  cent,  of  zinc  and  70  of  mer- 
cury the  conduction  is  greater  at  80°  C.  than  at  15°  C., 
and  remarks  that  we  must  remember  that  the  rate  of 
increase  of  conductivity  with  temperature  for  electro- 
lytes diminishes  as  the  concentration  increases,  and  that 
therefore  no  sharp  line  of  demarcation  can  be  drawn 
between  the  two  classes  of  conductors  on  this  account. 

In  1856  "W.  Weber  and  K.  Kohlrausch  endeavoured 
to  obtain  a  mechanical  measure  for  the  strength  of  an 
electrical  current,  and  to  this  end  they  studied  the 
electrolytic  decomposition  of  water,  and  stated  their  re- 
sults as  follows :  "  If  all  the  particles  of  hydrogen  in 
one  milligramme  of  water  contained  in  a  column  of  the 
length  of  one  millimeter  were  attached  to  a  string,  the 
particles  of  oxygen  being  attached  to  another  string,  each 
string  would  have  to  be  under  a  tension,  in  a  direction 
opposite  to  that  of  the  other,  of  2,956  hundredweight 
(147,830  kilogrammes),  in  order  to  effect  a  decomposi- 
tion of  the  water  with  a  velocity  of  one  milligramme 
per  second.* 

In  the  voltaic  arc  we  meet  with  phenomena  which 
are  analogous  to  the  actions  which  take  place  in  the 
voltaic  cell.  There  is  a  difference  of  molecular  activity 
at  the  surface  of  the  two  carbons.  The  positive  pole  be- 
comes much  hotter  than  the  negative  pole  and  burns 
away  twice  as  fast.  The  transformation  of  electricity 
into  heat  is  utilized  in  a  simple  process  of  electric  weld- 
ing, which  depends  on  the  phenomenon  of  the  formation 

*  Stallo,  Modern  Physics,  p.  307 ;  Pogg.  Ann.,  vol.  xcix,  p.  24 


THE  VOLTAIC  CELL.  33 

of  the  voltaic  arc.  The  positive  carbon  connected  with 
the  positive  pole  of  a  large  battery  or  a  dynamo  is  im- 
mersed in  a  suitable  conducting  liquid,  while  the  wire 
from  the  negative  pole  is  connected  with  the  rod  to 
which  another  rod  is  to  be  welded.  The  instant  the  rod 
thus  connected  with  the  negative  pole  touches  the  sur- 
face of  the  liquid  intense  heat  is  developed.  This  heat 
throws  the  liquid  into  the  spheroidal  state,  and  increases 
the  resistance  between  the  liquid  surface  and  that  of  the 
metal,  and  raises  the  latter  at  the  point  where  it  touches 
the  liquid  to  a  white  heat.  The  rod  to  be  welded  to 
the  rod  constituting  the  negative  pole  is  then  forced 
into  contact  with  the  latter.  The  rod  constituting  the 
negative  pole  can  be  held  in  the  hand  at  one  extremity 
while  the  other  is  at  a  white  heat,  so  quickly  is  the  tem- 
perature raised  at  the  end  which  touches  the  liquid. 
The  great  heat  of  the  voltaic  arc  is  utilized  also  in  the 
electric  furnace  which  is  employed  in  the  production 
of  aluminum,  and  in  various  other  processes  in  the  arts. 
This  furnace  consists  of  a  positive  and  negative  elec- 
trode of  carbon  in  the  interior  of  a  suitable  non-con- 
ducting crucible.  The  voltaic  arc  is  produced  between 
the  carbons  in  the  medium  which  is  to  be  reduced. 
Another  form  of  this  furnace  consists  of  the  two  elec- 
trodes of  carbon  immersed  in  a  mixture  of  carbon  and 
insulating  earth.  In  this  mixture  is  placed  a  crucible 
containing  the  substance  to  be  melted ;  the  furnace  is 
surrounded  by  a  layer  of  non-conducting  material. 

The  street  lamp  between  two  pieces  of  carbon  is  an 
electrical  furnace  without  the  insulating  covering.  We 
generally  think  of  the  current  flowing  from  the  positive 
pole  to  the  negative  pole.  If  we  use  an  alternating  cur- 
rent both  carbons  burn  at  the  same  rate.  Have  we  not, 
therefore,  in  this  phenomenon  direct  evidence  that  there 


81  WHAT  IS  ELECTRICITY! 

is  an  actual  flow  toward  the  negative  pole  ?  Let  us  ex- 
amine more  closely  the  formation  of  the  voltaic  arc.  In 
order  to  produce  it,  we  must  bring  the  positive  and  nega- 
tive carbon  into  contact  and  then  slowly  separate  them. 
The  resistance  of  the  thin  layer  of  air  between  the 
electrodes  is  broken  down  by  a  minute  spark  due  to  the 
difference  of  electro-motive  force  of  the  carbons  (about 
50  volts),  and  through  the  heated  air  the  voltaic  arc  is 
established.  Numerous  experiments  show  that  the  light 
of  the  arc  is  due  to  the  incandescence  of  particles  of  car- 
bon thrown  off  from  the  poles.  The  heated  air  is  prac- 
tically dark. 

In  our  discussion  of  the  phenomena  of  the  electric 
current  we  have  hitherto  fixed  our  minds  upon  the  wire, 
the  liquids  through  which  the  current  appears  to  flow, 
and  the  earth  which  forms  the  return  circuit.  Our  only 
consideration  of  the  air  has  been  in  the  phenomenon  of 
the  voltaic  arc  and  in  the  spark  discharge,  such  as  is 
witnessed  in  lightning.  Moreover,  we  have  regarded 
the  coating  of  a  wire  as  an  inert  thing  having  nothing  to 
do  with  what  we  have  regarded  as  a  flow  of  something. 
"We  have  separated  all  bodies  into  two  classes — conduc- 
tors and  insulators.  Our  telegraph  lines  are  strung  on 
glass  insulators.  The  wires  conducting  electricity  into 
our  houses  are  insulated  carefully  so  that  the  current 
may  not  leak  away.  It  would  be  a  strange  thought, 
therefore,  to  most  of  us  to  think  of  the  air,  the  glass,  and 
the  covering  of  the  wire  as  better  conductors  of  elec- 
trical energy  than  the  copper  conductor.  Yet  the  tend- 
ency of  modern  investigation  is  to  believe  in  this  hypo- 
thesis. 

According  to  this  hypothesis,  due  to  Poynting,  the 
electrical  energy  from  the  battery  or  the  dynamo  spreads 
out  into  space  and  decays  into  heat  when  it  encounters 


THE  VOLTAIC   CELL.  85 

the  wire  connecting  the  dynamo  at  the  central  station 
with  the  motor  in  the  electric  car.  It  may  be  said  to 
converge  upon  the  motor  in  the  car,  and  is  there  recon- 
verted into  motion.  The  flow  of  current,  or  the  rate  of 
electrification  along  the  wire,  is  not  hi  the  same  direc- 
tion as  the  flow  of  electrical  energy  in  the  space  not  filled 
with  the  wire.  We  see  here  that  the  water  analogies  do 
not  aid  us,  for  the  water  pumped  in  at  one  end  of  a 
pipe  transmits  along  the  interior  of  the  pipe  the  energy 
that  is  exerted  at  the  transmitting  end.  None  of  the 
energy  of  the  transmitter  which  in  this  case  is  the 
pump  is  transmitted  through  the  space  outside  the  pipe. 


CHAPTER  VII. 

THE   GALVANOMETER. 

OUR  general  survey  of  the  phenomena  of  the  elec- 
tric current  presents  the  great  main  features  of  the 
subject  of  electricity  as  it  was  known  to  the  world  in 
1830,  if  we  except  the  phenomenon  of  the  spreading 
out  of  the  electric  current  in  the  earth  and  the  use  of 
the  earth  as  a  return  circuit.  The  knowledge  of  the 
action  of  a  battery  in  the  early  part  of  this  century 
strongly  resembles  the  state  of  the  world's  knowledge 
of  the  action  of  the  human  heart  in  1630.  Two  hun- 
dred years  previous  to  1830  Harvey  had  shown  that 
the  arterial  blood  flowed  out  from  the  heart,  and,  being 
converted  into  venous  blood,  flowed  back  again.  The 
heart  of  men  and  animals  was,  like  the  battery,  the 
mysterious  source  of  a  strange  circulation  which  was 
followed  only  imperfectly  by  Harvey,  for  he  needed 
finer  and  more  subtle  means  of  tracing  the  entire  extent 
of  this  circulation  through  the  minute  vessels  which 
are  called  capillaries.  Of  one  thing,  at  least,  he  was 
certain :  the  source  of  the  circulation  was  the  action  of 
the  heart,  and  in  the  dedication  of  his  book  on  the  cir- 
culation of  the  blood  to  Charles  I  he  reminds  this  illus- 
trious prince  that,  "as  the  heart  of  animals  is  the 
foundation  of  their  life,  the  source  of  everything  within 
them,  the  sun  of  their  microcosm,  that  upon  which  all 


THE  GALVANOMETER.  87 

growth  depends,  from  whom  all  power  proceeds,  the 
king  in  like  manner  is  the  foundation  of  his  king- 
dom, the  sun  of  the  world  around  him,  the  heart  of  his 
republic,  the  fountain  whence  all  power,  all  grace  doth 
flow." 

It  was  not  until  1690  that  Leuwenhoek  showed  by 
means  of  the  microscope  that  "  the  blood  passes  from 
the  arteries  into  the  veins  by  a  network  of  minute  ves- 
sels, the  thin  walls  of  which  allow  the  fluid  plasma  to 
transude  into  the  tissues  so  as  to  serve  for  their  nutri- 
tion." 

When  Harvey  explained  his  theory  of  the  circula- 
tion of  the  blood  to  Charles  I,  the  king  must  have  re- 
garded the  heart  much  as  the  educated  man  to-day, 
ignorant  of  electricity,  regards  the  power  house  from 
which  the  electrical  current  apparently  proceeds  and 
returns  through  the  bosom  of  the  earth,  he  knows  not 
how.  Indeed,  Harvey,  although  he  was  certain  of  the 
main  facts,  was  at  a  loss  to  explain  how  the  blood  got 
from  the  arteries  to  the  veins.  A  delicate  instrument 
was  needed  to  help  the  human  eye,  and  in  the  hands  of 
Leuwenhoek  the  microscope  proved  to  be  this  instru- 
ment. This  little  instrument  has  greatly  extended  our 
knowledge  of  the  immense  activities  which  result  from 
the  action  of  the  heart.  When,  too,  Yolta  was  sum- 
moned to  Paris  to  explain  the  electric  battery  to  the 
great  Napoleon,  the  emperor  must  have  regarded  the 
voltaic  cell  as  a  heart,  from  which  a  mysterious  flow 
proceeded,  no  one  knew  why  or  how.  Yolta  himself 
probably  never  suspected  that  pulsations  in  the  action 
of  the  cell  could  be  detected  by  an  electrical  microscope 
in  all  neighbouring  masses  of  metal,  and  indeed  in  space 
itself.  This  electrical  microscope  is  termed  a  galvanom- 
eter, and  it  plays  a  part  in  electrical  science  very  simi- 


88  WHAT  IS  ELECTRICITY  1 

lar  to  that  enacted  by  the  microscope  in  medical  science. 
The  microscope  and  the  galvanometer  illustrate  what 
immense  advances  in  our  knowledge  can  be  made  by 
suitable  instruments.  By  means  of  the  galvanometer, 
Michael  Faraday  and  Joseph  Henry  discovered  the 
principle  of  the  dynamo  machine. 

It  is  a  strange  reflection  that  these  philosophers 
could  have  made  their  discoveries  by  merely  employing 
the  microscope,  using  the  agency  of  light  to  discover 
the  manifestations  of  electricity.  We  have  said  that  a 
compass  instantly  points  to  a  wire  through  which  a 
current  of  electricity  is  passing,  and  that  fine  magnetic 
filings  cling  to  the  wire.  By  distributing  iron  dust  on 
the  stage  of  a  microscope  near  a  wire  through  which  a 
current  of  electricity  is  made  to  pulsate,  an  observer 
looking  at  the  particles  of  dust  through  the  microscope 
will  see  them  pulsate  also.  Furthermore,  by  winding 
two  little  bobbins  with  fine  insulated  wire,  or,  in  other 
words,  making  little  wire  spools,  similar  to  small  spools 
of  thread,  connecting  the  two  ends  of  the  wire  on  one 
spool  to  the  ends  of  the  wire  on  the  other  spool,  plac- 
ing one  spool  on  the  stage  of  the  microscope  with  iron 
filings  on  a  piece  of  paper  laid  on  its  end,  and  placing 
the  other  spool  on  a  third  spool  made  of  coarse  wire  the 
ends  of  which  are  connected  with  a  battery,  one  can  ob- 
serve that  when  the  battery  current  is  suddenly  made  or 
suddenly  broken  the  little  particles  of  iron  vibrate,  show- 
ing the  existence  of  currents  of  electricity  in  the  circuit 
of  wire  on  the  two  connected  spools.  This  is  the  great 
discovery  of  currents  of  induction,  made  by  Henry  and 
Faraday,  and  it  is  the  foundation  of  the  action  of  the 
dynamo  and  of  the  telephone.  It  shows  that  any 
change  in  an  electric  current  on  a  wire,  any  pulsation, 
causes  instantly  a  similar  pulsation  in  any  neighbouring 


THE  GALVANOMETER.  89 

wire  not  connected  with  the  first  wire  and  placed  paral- 
lel to  it. 

One  of  the  principal  objections  to  using  the  micro- 
scope in  this  manner  resides  in  the  friction  between  the 
fine  particles  of  iron  which  tends  to  prevent  their  free 
motion.  A  better  way  is  to  suspend  a  very  fine  cam- 
bric needle  from  its  middle  by  a  spider  thread  or  a  bit 
of  coccoon  fibre  drawn  from  a  white  silk  thread.  Make 
the  needle  a  magnet  by  allowing  it  to  rest  for  a  moment 
across  the  poles  of  a  strong  horseshoe  magnet ;  place  the 
little  bobbin  of  wire  not  on  its  end,  but  on  its  side,  on 
the  stage  of  the  microscope,  and  bring  one  end  of  the 
needle  near  the  end  of  the  bobbin  and  focus  the  micro- 
scope upon  this  end.  The  needle  should  be  protected 
from  currents  of  air.  It  will  be  a  compass  pointing 
north  and  south  and  the  ends  of  the  bobbin  should  lie 
east  and  west.  This  microscopic  galvanometer  can  be 
made  to  detect  very  feeble  currents  of  electricity,  and 
will  show  that  any  change  of  strength  of  an  electric  cur- 
rent in  the  bobbin  connected  with  the  battery  will  mani- 
fest itself  in  a  neighbouring  bobbin  in  a  circuit  of  wire 
totally  disconnected  and  independent  of  the  battery  cir- 
cuit. Furthermore,  the  extent  of  movement  of  the 
needle  over  a  suitably  graduated  scale  of  fine  divisions 
will  measure  the  electrical  strength  of  the  impulses 
given  to  the  needle.  The  microscope  thus  magnifies 
the  motion  of  the  magnet.  Henry  and  Faraday,  how- 
ever, were  ignorant  of  the  use  of  a  microscope  for  the 
detection  of  the  very  feeble  currents  which  afterward 
were  exalted  into  the  tremendous  currents  which  drive 
our  electric  cars  and  light  our  cities,  and  used  the  unas- 
sisted eye  to  observe  the  motion  of  a  fine  magnetized 
cambric  needle,  one  pole  of  which  was  opposite  the  end 
of  a  bobbin  or  spool  covered  with  many  turns  of  very 


90  WHAT  IS  ELECTRICITY? 

fine  insulated  wire.  The  longer  the  needle  in  general, 
the  larger  the  deflections.  In  order  to  avoid  weight  in 
the  suspended  magnet  it  was  made  of  a  bit  of  magnet- 
ized steel,  which  was  provided  with  very  light  hairlike 
pointers.  The  ends  of  these  pointers  moved  over  a 
graduated  scale.  To-day,  instead  of  pointers,  a  long 
beam  of  light  is  used  as  an  index.  It  has  no  weight, 
and  it  takes  the  place  of  pointers  at  least  three  feet  long. 
To  use  a  beam  of  light  the  suspended  magnet  is  stuck 
upon  a  tiny  mirror  of  very  thin  silvered  glass  and  a  beam 
of  light  is  reflected  from  this  mirror  upon  a  distant 
scale  or  into  a  suitable  telescope.  A  small  movement 
of  the  magnet  thus  results  in  a  large  movement  of  the 
beam  of  light  on  the  distant  scale.  This  is  a  method  of 
magnifying  small  movements  which  is  much  used  in 
physics,  and  it  has  even  a  practical  application,  for  it 
showed  the  possibility  of  telegraphing  beneath  the 
ocean  by  means  of  the  Atlantic  cable,  and  it  has  been 
constantly  in  commercial  use  on  the  cable  up  to  this 
date.  The  device  of  a  tiny  mirror  on  a  suspended  mag- 
net, applied  by  Lord  Kelvin,  has  been  of  great  prac- 
tical use  in  submarine  telegraphy.  The  microscope 
made  possible  the  investigations  which  have  led  to  the 
germ  theory  of  disease,  the  antiseptic  treatment  in  sur- 
gery, and  founded  the  subject  of  physiological  botany. 
The  galvanometer  has  shown  that  electrical  actions  per- 
vade all  matter,  and  that  there  are  electric  waves  in  the 
ether  of  space. 

With  a  delicate  galvanometer  the  field  of  our  knowl- 
edge of  electrical  activity  is  enormously  increased.  We 
immediately  discover  that  the  slightest  movement  of  a 
magnet  near  a  wire  is  accompanied  by  an  electrical  dis- 
turbance on  the  wire.  If  we  thrust  one  pole  of  a  mag- 
net into  a  spool  covered  with  wire  the  ends  of  which 


THE   GALVANOMETER.  91 

are  connected  with  the  terminals  of  wire  wound  on  a 
similar  spool,  a  little  magnet  provided  with  a  mirror, 
and  suspended  opposite  the  end  of  the  latter  spool,  will 
show  a  momentary  electric  current  in  the  electric  cir- 
cuit constituted  by  the  two  connected  bobbins.  It 
dies  out  immediately,  but  it  is  instantly  renewed  when 
we  pull  out  the  pole  of  the  magnet  instead  of  thrust- 
ing it  into  the  bobbin.  When  we  pull  the  pole  out, 
the  suspended  magnet  moves  in  one  way,  and  when  we 
thrust  it  in,  it  moves  in  an  opposite  way.  Furthermore, 
if  we  thrust  the  south  pole  of  a  magnet  into  the  bobbin 
the  galvanometer  needle  moves  in  one  direction,  and 
when  we  thrust  in  the  north  pole  it  moves  in  another 
direction.  Since  we  perceive  that  any  movement  of  a 
magnet  near  a  coil  of  wire  is  attended  by  an  electrical 
disturbance  in  the  wire,  it  is  natural  to  suppose  that  we 
can  produce  electrical  disturbances  by  keeping  the  mag- 
net fixed  in  position  and  by  moving  the  coil  of  wire. 
Experiment  will  speedily  verify  this  conclusion.  TTe 
find  that  a  movement  of  our  little  bobbin  or  coil  of  wire 
in  the  air  near  a  pole  of  a  magnet  excites  currents  in  the 
circuit  connected  with  the  bobbin. 

Since  the  earth  appears  to  be  a  magnet,  we  should 
expect,  therefore,  to  find  that  if  we  move  our  little  coil 
in  the  air  we  should  obtain  an  electrical  current.  Ex- 
periment shows  this  to  be  true,  and  it  still  further  illus- 
trates what  the  delicate  galvanometer  revealed  to 
Faraday  and  Henry.  There  was  something  in  the  space 
outside  a  magnet  which  could  be  made  evident  by  mov- 
ing either  a  wire  or  the  magnet.  The  slightest  change 
in  the  current  passing  through  one  wire  makes  itself 
manifest  across  space  in  a  distant  wire.  The  galvanom- 
eter can  detect  slight  molecular  disturbance ;  it  can 
also  reveal  mysterious  effects  in  the  ether  of  space.  One 


92  WHAT  IS  ELECTRICITY! 

sometimes  smiles  at  the  microscopist  who  disputes  with 
a  fellow-microscopist  on  the  possibility  of  measuring 
spaces  of  one  hundred  thousandth  of  an  inch.  Yet  one 
should  reflect  that  the  antiseptic  treatment  in  surgery, 
which  saves  thousands  of  lives  every  year,  is  due  to  the 
perfection  of  the  microscope.  To  those  unlearned  in 
electrical  science,  too,  the  tiny  movement  of  a  needle  or 
the  excursion  of  a  spot  of  light  over  a  scale  seems  hardly 
worthy  the  attention  of  a  liberally  educated  man.  Yet 
the  modern  dynamo  and  the  telephone  owe  their  exist- 
ence to  the  study  by  Faraday  and  Henry  of  these 
minute  movements. 

I  have  endeavoured  to  explain  the  construction  of 
the  galvanometer,  which  I  have  termed  the  electrical 
microscope,  and  in  the  following  chapter  I  shall  en- 
deavour by  its  aid  to  explain  the  action  of  the  dynamo 
machine  and  the  electrical  motor  which  is  now  used  to 
propel  electric  cars.  The  galvanometer,  we  have  seen, 
consists  in  its  essentials  of  a  coil  or  bobbin  of  wire 
like  a  spool  of  thread  with  a  tiny  magnet  hung  by  a 
spider  thread  near  one  end  of  the  spool.  When  a  cur- 
rent of  electricity  flows  through  the  spool  it  makes  the 
spool  an  electro-magnet  with  two  poles,  and  the  pole 
near  one  end  of  the  spool  consequently  attracts  one  pole 
of  the  little  suspended  magnet  and  repels  the  other,  thus 
causing  the  tiny  magnet  to  turn  around  its  axis  of  sus- 
pension. When  the  current  ceases  the  suspended  mag- 
net returns  to  its  original  position,  pointing  north  and 
south.  The  amount  of  its  turn  measures  the  strength 
of  the  electro-magnet. 

There  is  another  instrument  called  the  electrometer, 
which  is  used  by  electricians  to  measure  the  electro- 
motive force  instead  of  the  current.  The  galvanometer 
indicates  the  current  which  results  from  a  certain  electro- 


THE  GALVANOMETER.  93 

motive  force ;  it  does  not  measure  directly  the  electro- 
motive force  which  gives  rise  to  the  current.  For  in- 
stance, it  will  measure  the  current  given  by  a  silver 
spoon  and  an  iron  spoon  immersed  in  a  tumbler  of  water, 
but  it  does  not  tell  us  directly  what  is  the  electro-motive 
force  between  the  iron  and  the  silver.  This  electro- 
motive force  is  similar  to  the  pressure  of  the  water  or 
the  steam  at  the  power  house,  under  which  pressure  the 
circulation  is  caused  in  the  pipes  issuing  from  the  power 
house.  It  has  been  found  that  any  two  metals  immersed 
in  a  liquid  which  acts  unequally  upon  them  give  rise  to 
an  electro-motive  force,  which  varies  in  strength  with 
the  character  of  the  metals.  It  is  therefore  useful  to 
have  an  instrument  which  will  serve  as  a  sort  of  gauge 
of  electrical  pressure.  This  instrument  is  called  a  volt- 
meter, the  volt  being  the  unit  of  electro-motive  force. 
The  indications  of  this  instrument  are  graduated  by 
means  of  what  is  called  the  Latimer  Clark  cell.  This 
is  a  voltaic  cell  which  produces  a  constant  electro-motive 
force  and  which  therefore  serves  as  a  unit. 


CHAPTER  VIII. 

THE   DYNAMO   MACHINE. 

WE  have  remarked  that  the  ancients  could  not  have 
possessed  dynamo  machines  or  telephones,  for  they  were 
ignorant  of  the  art  of  covering  wire  by  cotton  or  silk, 
and  it  is  doubtful  whether  they  knew  the  art  of  draw- 
ing wire.  If  by  any  cataclysm  or  widespread  catastro- 
phe the  European  nations  should  disappear  from  the 
face  of  the  earth  and  some  East  Indian  tribe  ignorant 
of  electricity  should  alone  survive,  and  through  the 
slow  ages  rise  and  possess  the  American  continent,  their 
archaeologists  might  find  the  ancient  ruins  filled  with 
copper  wires.  There  is  no  trace  of  such  wires  in  the 
ruins  of  Egypt  or  of  Greece.  In  reading  the  Life  of 
Faraday,  and  his  account  of  his  discovery  of  the  princi- 
ple underlying  the  great  advances  in  electricity,  we 
wonder  why  the  phenomena  which  he  discovered  had 
not  been  discovered  before,  and  by  men  with  less  men- 
tal equipment.  When  we  consider,  however,  the  form 
of  galvanometer  with  which  he  worked,  and  the  limited 
supply  of  insulated  copper  wire  of  different  degrees  of 
fineness  which  was  at  his  disposal,  our  wonder  disap- 
pears. With  a  modern  galvanometer  a  senior  in  Har- 
vard University  of  fair  intelligence  could  not  fail  to 
discover  the  laws  of  magnetic  induction,  for  an  acci- 
dental movement  of  a  magnet  near  a  coil  of  wire  would 


THE  DYNAMO  MACHINE.  95 

certainly  reveal  these  laws.  Faraday's  merit  consisted 
not  so  much  in  the  discovery  of  the  phenomena  as  in 
his  mental  conception  of  lines  of  force  pervading  all 
space. 

It  will  be  interesting,  therefore,  to  compare  Fara- 
day's instruments  with  the  more  sensitive  modern  ap- 
paratus. In  the  first  place,  in  common  with  Tyndall, 
he  used  a  form  of  needle  galvanometer,  and  he  observed 
the  movements  of  the  ends  of  the  needles  over  a  gradu- 
ated circle  just  as  we  observe  in  a  pocket  compass  the 
movement  of  the  needle  of  the  compass  over  the  divi- 
sions of  the  circle  placed  beneath  the  end  of  the  needle. 
It  is  evident  that  a  small  angular  movement  of  the 
needle  can  not  be  very  much  magnified  on  a  circle  of 
small  dimensions.  Faraday's  circle  was  not  more  than 
four  inches  in  diameter.  In  the  modern  galvanometers 
what  would  correspond  to  circles  of  ten  feet  in  diameter 
are  often  used,  thirty  times  the  diameter  of  Faraday's 
scale.  Let  us  examine  further  Faraday's  galvanometer 
and  the  modern  form,  remembering  that  we  are  now 
dealing  with  an  electrical  microscope  which  has  revealed 
the  great  world  of  electrical  activity  in  which  we  move. 

Faraday's  and  Henry's  galvanometer  consisted,  in 
the  first  place,  of  two  ordinary  needles — cambric  nee- 
dles— which  were  magnetized  and  fixed  one  above  the 
other  (Fig.  8)  to  a  rigid  bar,  A  B.  The  poles  of  one 
needle  were  opposed  to  those  of  the  other — that  is,  a 
north  pole  was  placed  above  a  south  pole,  or  a  south 
pole  above  a  north  pole.  The  object  of  this  arrange- 
ment was  to  weaken  the  effect  of  the  north  pole  of  the 
earth,  so  that  the  arrangement  of  needles  should  not  be 
held  so  strongly  to  the  north  and  south  direction,  and 
should  yield  to  a  very  slight  attracting  force  at  right 
angles  to  the  direction  of  the  earth's  pull  on  the 


96 


WHAT  IS  ELECTRICITY! 


compass.  The  two  needles  were  suspended  to  a  fixed 
support,  C,  by  means  of  a  strand  of  cocoon  fibre.  This 
arrangement  will  show  the  presence  of  an  ordinary  bar 
magnet  five  or  six  feet  away.  If  only  one  needle  is 
used,  it  will  not  show  the  attracting  force  of  the  same 
bar  magnet  if  the  latter  is  brought  within  three  feet  of  it, 
so  strongly  would  one  needle  be  held  by  the  north  and 
south  poles  of  the  earth. 
Beside  Faraday's  little  sus- 
pended magnet  I  have  rep- 
resented in  Fig.  9  the  sus- 
pended magnets  in  a  mod- 
ern galvanometer.  At  the 
end  of  a  light  aluminium 
or  rigid  glass  fibre  are 
placed  collections  of  tiny 
magnets.  The  north  poles 
of  those  at  A  are  opposed 
to  the  south  poles  of  those 
at  B,  and  the  south  poles 


FIG.  8. 


_    B 
:S 
FIG.  9. 

at  A  are  opposite  the  north 

poles  at  B.  A  little  mirror  of  very  thin  silvered  glass 
is  placed  at  N".  The  weight  of  Faraday's  suspended  ap- 
paratus was  at  least  400  milligrammes,  while  the  weight 
of  the  modern  form  is  only  80  milligrammes— hardly 
more  than  that  of  a  butterfly's  wing.  Moreover,  the 
later  form  is  suspended  from  the  fixed  support  by  a 
quartz  fibre  finer  than  the  finest  hair.  This  quartz  fibre 
is  very  strong  and  possesses  hardly  any  torsion — that  is, 
any  power  to  resist  a  twist.  The  suspended  magnets 
will  therefore  turn  about  the  axis  of  their  suspension 
under  the  influence  of  the  slightest  attracting  force. 
The  use  of  this  quartz  fibre,  due  to  Prof.  C.  V.  Boys, 
constitutes  a  great  improvement  in  the  suspended  ap- 


THE  DYNAMO  MACHINE.  97 

paratus  of  the  galvanometer.  It  is  obtained  in  the 
following  manner :  A  bit  of  quartz  is  suitably  attached 
to  the  arrow  of  a  crossbow.  It  is  then  melted  in  an 
oxyhydrogen  blowpipe,  which  fuses  it  into  a  globule  of 
quartz  glass.  At  the  proper  point  of  fusion  the  cross- 
bow is  discharged  and  the  arrow  flies  away,  carrying  a 
fine  thread  of  the  melted  quartz.  Threads  of  different 
degrees  of  fineness  can  be  obtained  by  regulating  the 
strength  of  the  crossbow. 

The  suspended  apparatus,  therefore,  of  the  modern 
galvanometer  is  at  least  five  times  lighter  than  that  used 
by  Faraday.  It  will  turn  about  its  axis  with  far  greater 
readiness  under  the  influence  of  slight  attraction,  such 
as  is  exerted  by  a  distant  magnet.  But  the  greatest 
improvement  consists  in  magnifying  its  movement 
around  its  axis  of  suspension  by  means  of  the  reflection 
of  a  spot  of  light  from  the  tiny  mirror.  By  means  of 
such  a  reflection  we  can  practically  increase  Faraday's 
graduated  circle  of  four  inches  in  diameter  to  a  circle 
of  one  hundred  and  twenty  inches  in  diameter ;  and, 
moreover,  we  use  as  a  long  index  a  beam  of  light,  with- 
out weight.  A  suspended  apparatus  of  this  form  will 
show  the  presence  of  a  bar  magnet  at  a  distance  of  at 
least  thirty  feet.  In  the  Jefferson  Physical  Laboratory 
it  responds  to  the  passage  of  the  electric  cars  four  hun- 
dred feet  away. 

In  both  the  earlier  and  later  forms  of  galvanometer 
the  lower  suspended  magnets  are  hung  at  the  centre  of 
little  coils  of  insulated  wire  of  many  turns.  When  an 
electric  current  flows  through  the  wire  of  the  coils  it 
makes  the  coils  an  electro-magnet,  and  the  suspended 
magnets  tend  to  place  themselves  in  the  axis  of  the  coils 
— that  is,  their  poles  are  attracted  to  the  poles  of  the 
electro-magnet  The  attraction  of  the  earth  tends  to 


98  WHAT  IS  ELECTRICITY! 

turn  the  magnets  back  into  the  north  and  south  line,  or 
into  the  magnetic  meridian,  and  they  therefore  take  up 
an  intermediate  position,  and  this  position  serves  as  a 
measure  of  the  force  of  the  electric  current  in  the  coils. 

A  lady's  workbasket,  with  the  addition  of  a  toy 
magnet  and  with  fine  covered  wire  instead  of  thread, 
affords  all  the  material  for  illustrating  the  great  prin- 
ciples of  magneto-induction,  which  underlie  the  action 
of  the  dynamo  machine  and  the  telephone.  Two  cambric 
needles  can  be  magnetized  by  placing  them  for  a  mo- 
ment upon  the  poles  of  the  toy  horseshoe  magnet  so  that 
they  constitute  its  armature.  They  can  then  be  stuck, 
after  the  manner  of  the  needles  in  Faraday's  galvanom- 
eter, through  a  bit  of  light  wood ;  and  the  two  needles 
can  be  suspended  by  the  ultimate  fibre  drawn  from  a 
white  silk  thread.  A  spool  wound  with  fine  silk-covered 
wire  instead  of  thread  can  be  placed  with  one  end  oppo- 
site one  pole  of  one  of  the  needles,  and  we  shall  have 
then  a  model  of  a  Faraday  galvanometer  in  all  essential 
respects.  The  pole  of  the  cambric  needle  will  turn 
toward  the  end  of  the  spool  of  wire  when  a  sufficiently 
powerful  current  circulates  through  this  spool,  and  by 
its  movement  will  thus  indicate  the  strength  of  this 
current. 

Suppose  that  in  the  adjoining  Fig.  10,  !N"  S  repre- 
sent the  lower  suspended  cambric  needle  as  it  appears 
when  we  look  directly  down  the  suspending  fibre,  and 
that  S  represents  the  spool  covered  with  wire.  Let  an- 
other similar  spool,  S',  be  placed  at  some  distance,  and 
let  the  ends  of  its  wire,  B,  D,  be  connected  with  the 
ends  of  the  wire,  A,  C,  of  the  first  spool.  If  we  should 
now  thrust  the  north  pole  of  a  thick  bundle  of  mag- 
netized needles  into  the  coil  S'  the  little  needle  !N"  S 
will  move  in  one  direction,  and  when  the  magnet,  K 


THE  DYNAMO  MACHINE. 


99 


S,  is  pulled  out  of  the  coil  it  will  move  in  a  reverse 
direction.  If,  therefore,  before  we  pull  out  the  magnet 
we  also  reverse  the  connections  of  the  wires — that  is, 
connect  D  to  A  and  C  to  B  or  the  reverse — the  needle 
will  always  move  in  the  same  direction  whether  we 
thrust  the  pole  K  into  the  spool  S'  or  pull  it  out.  The 
person  who  thus  assists  the  experiment  by  changing  the 
connections  of  the  ends  of  the  wires  can  be  called  a 
commutator.  He  commutes  the  direction  of  the  cur- 


A  B 


C  D 


FIG.  10. 

rents  which  fly  in  opposite  directions  through  the  spool 
S',  so  that  they  shall  always  pass  through  the  spool  S  in 
the  same  direction,  and  so  that  we  shall  thus  have — if 
he  commutes  swiftly  enough — a  steady  current  through 
S  instead  of  a  to-and-f  ro  or  alternating  current.  It  is 
evident  that  it  would  require  very  little  ingenuity  on  the 
part  of  the  person  who  acts  as  commutator  to  arrange  a 
mechanical  apparatus  which  would  relieve  him  of  his 
task  and  would  commute  automatically — that  is,  place 
the  ends  A,  B,  and  C,  D,  in  communication  when  the 
pole  N  is  thrust  into  S',  and  the  ends  A,  D,  and  C,  B, 
when  it  is  withdrawn.  There  is  an  interesting  analogy 
between  the  action  of  the  automatic  commutator  and  the 
eccentric  of  the  steam  engine,  which  admits  steam  first 
at  one  end  of  the  piston  and  then  at  the  other.  In  the 
early  forms  of  the  steam  engine  which  were  employed 
in  the  mining  districts  in  England  steam  was  admitted 
beneath  a  piston  by  turning  a  cock  by  hand.  The  steam, 


100  WHAT  IS  ELECTRICITY? 

having  lifted  a  load  by  means  of  the  piston,  was  dis- 
charged from  the  cylinder  so  that  it  might  move  back 
ready  again  for  an  upward  stroke.  It  is  said  that  the 
boy  who  had  charge  of  admitting  the  steam  properly 
devised  the  eccentric  to  lessen  his  labours — to  do  the 
work  for  him  while  he  occupied  his  leisure  in  playing 
marbles.  This  story  may  be  a  fable,  but  it  serves  our 
purpose  well  for  an  illustration.  The  eccentric,  it  is 
known,  is  a  mechanism  connected  with  the  moving 
parts  of  the  piston  rod,  which  reverses  the  admission  of 
steam  from  one  side  of  the  piston  to  the  other,  in  order 
to  secure  a  continuous  movement  to  and  fro  of  the  pis- 
ton rod.  The  eccentric  makes  the  modern  steam  engine 
a  practical  continuously  moving  machine,  and  the  elec- 
tric commutator  has  made  possible  the  modern  dynamo 
and  electric  motor.  I  dwell  at  some  length,  therefore, 
upon  this  simple  practical  detail,  to  illustrate  how 
greatly  the  advance  of  science  depends  upon  the  ad- 
vance of  the  mechanic  arts.  The  machines  for  cover- 
ing copper  wire  and  the  perfection  of  the  galvanometer 
and  commutator  have  made  possible  the  rapid  practi- 
cal employment  of  the  feeble  currents  discovered  by 
Faraday.  The  commutator  has  exalted  them  from  cur- 
rents which  were  barely  perceptible  to  Faraday  to  cur- 
rents capable  of  driving  electric  cars  weighing  many 
tons. 

I  have  employed  the  following  simple  apparatus  to 
illustrate  the  action  of  the  dynamo  and  its  commutator, 
whiclrrequires  very  little  mechanical  skill  to  construct. 
A  disk  of  wood  about  a  foot  in  diameter  (Fig.  HA)  has 
a  pointed  stick  driven  through  its  centre,  thus  mak- 
ing a  large  Japanese  top.  The  disk  being  very  near 
one  end  of  the  rod  which  passes  through  its  centre, 
such  a  top  will  continue  to  revolve  in  the  same  spot  on 


THE  DYNAMO  MACHINE. 


101 


a  smooth  floor  for  a  long  time  by  merely  twirling  the 
rod  between  the  fingers.  Stick  now  a  little  flat  spool 
of  insulated  copper  wires  (S')  upon  the  wooden  disk, 
lead  its  wires  up  the  rod,  and  bind  over  their  bare  ends 
tightly  two  little  shieldlike  pieces  of  bright  sheet  cop- 
per or  brass,  so  that  these  shields  partly  embrace  the 
rod,  being  insulated  from  each  other  by  the  wood. 


FIG.  HA. 


These  shields  form  the  ends,  B,  D,  of  the  moving  spool, 
S'.  On  a  piece  of  wood  in  which  a  hole  is  bored  to 
admit  of  the  top  turning  run  the  wires  A,  C,  to  the 
coil  M  of  the  galvanometer,  and  remove  their  covering, 
so  that  the  bare  wire  shall  touch  the  ends  B  and  D. 
When  the  top  revolves,  the  coil  S'  passes  under  the 
south  pole  S  of  a  horseshoe  magnet,  and  an  electric 
current  flies  through  the  spool  S'  in  the  direction  of 


102  WHAT  IS  ELECTRICITY? 

the  arrows  L  I/.  When  S'  passes  under  the  pole  N  a 
reverse  current  is  induced  in  it  in  the  direction  of  the 
arrows.  At  the  same  time,  however,  the  ends  of  the 
wires  B  and  D  are  reversed,  so  that  D  touches  A  and 
B  touches  C,  and  the  current  is  still  delivered  to  M  in 
the  same  direction. 

This  model  represents  the  modern  dynamo  machine 
in  certain  of  its  essential  features.  It  shows  that  cur- 
rents of  electricity  are  excited  in  a  coil  of  wire  passing 
rapidly  near  the  poles  of  a  magnet ;  that  a  current  is 
excited  in  the  coil  in  one  direction  by  movement  near  a 
south  pole,  and  in  the  opposite  direction  by  movement 
near  a  north  pole  ;  and  that  a  commutator  can  be  made 
to  direct  these  opposite  currents  in  the  same  direction 
through  another  coil.  The  next  great  advance  in  the 
dynamo  machine  was  in  a  simple  process  of  strengthen- 
ing the  poles  of  the  horseshoe  magnet  by  the  current 
which  it  excites  in  the  moving  coil,  S'.  This  was  done 
by  running  the  current,  which  we  have  shown  can  be 
tested  by  the  galvanometer,  M,  around  the  poles  of  the 
horseshoe  magnet,  N  and  S,  wrapping  the  wire  around 
the  pole  N  in  one  direction  and  around  the  pole  S  in 
the  opposite  direction,  so  as  to  excite  opposite  poles ; 
for  direct  experiment  shows  that  the  direction  of  the 
winding  about  a  piece  of  iron  or  steel  determines 
whether  a  north  or  a  south  pole  is  made.  By  means  of 
this  arrangement  of  the  circuit  a  feeble  current  in  the 
spool  S',  strengthens  the  poles  N"  and  S.  This  strength- 
ening in  turn  leads  to  a  stronger  current  in  the  coil  S', 
and  this  stronger  current  again  strengthens  still  further 
the  poles  N  and  S.  This  process  goes  on  like  the  law 
of  compound  interest,  until  the  work  done  in  twirling 
the  top  is  just  equal  to  the  work  done  in  overcoming  the 
repulsions  and  attractions  between  the  poles  of  the  little 


THE   DYNAMO  MACHINE. 


103 


moving  coil  S'  and  the  poles  of  the  electro-magnet  K"  S. 
That  there  exists  a  repulsion  instead  of  an  attraction  be- 
tween the  moving  coil  S'  as  it  approaches  the  poles  .N" 
and  S  is  evident  from  this  consideration.  Suppose  that 
an  attraction  should  exist.  After  the  top  is  once  started 
it  would  go  on  forever,  for  the  coil  S'  would  be  at- 
tracted to  the  south  pole,  S,  and  then  to  the  north  pole, 
N,  and  so  on.  Our  top  would  then  be  a  perpetual-mo- 
tion machine,  the  essence  of  perpetual -motion  machines 
being  hi  their  ability  to  move  without  consuming  any 
work. 

If  we  should  now  turn  our  top  by  a  steam  engine 
instead  of  by  the  fingers,  we  should  have  the  entire 
arrangements  of  a  modern  dynamo  machine.  Instead 
of  one  coil,  S',  we  should  have  a  number,  and  the  poles 
K  and  S  would  be  more  advantageously  placed.  Still, 
our  model  represents  in  its  essential  features  the  mod- 
ern dynamo,  and  also  the  electric  motor.  Fig.  HB 
shows  the  practical  arrangement  of  what  is  called  a 
series  dynamo, 
the  coils  on  an 
iron  ring  be- 
ing connected 
to  segments  of 
the  commuta- 
tor, X,  while 
the  current  is 
taken  off  by  the 
brushes,  b  b' ', 
and  is  made  to 

flow  around  the  FJQi 

field    magnets, 

N  and  S,  so  as  to  make  north  and  south  poles,  as  the 
figure  indicates.  It  is  well  to  note  here  that  a  steam 


104:  WHAT  IS  ELECTRICITY! 

engine  is  employed  to  drive  all  dynamos,  and  that  be- 
hind all  our  wonderful  uses  of  electricity  is  steam.  We 
see,  therefore,  that  electricity  can  not  supersede  steam 
as  a  source  of  power  until  some  method  of  obtaining 
electricity  direct  from  coal  is  discovered.  At  present 
there  is  apparently  no  feasible  method.  The  popular 
impression,  therefore,  that  electricity  will  speedily  su- 
persede steam  has  no  foundation  in  fact.  This  impres- 
sion may  be  prophetic,  but  steam  is  now  the  source  of 
all  our  electricity  in  the  transmission  of  power,  if  we 
except  water  power. 


CHAPTEK  IX. 

SOURCES   OF   ELECTRIC   POWEE. 

THE  practical  applications  of  electricity  afford  rich 
illustrations  of  the  transformations  of  energy.  The  coal 
which  generates  the  steam  which  drives  the  engine  was 
produced  by  electro-magnetic  waves  from  the  sun.  In 
the  dim  past  these  waves,  in  the  form  of  light  and  heat 
waves,  nourished  the  great  ferns  and  palm  trees,  and  all 
that  luxuriant  vegetation  which  now  constitutes  our 
chief  reliance  for  power.  Through  the  combustion  of 
the  coal  we  are  enabled  to  drive  the  dynamo  which  pro- 
duces the  electric  energy,  which  is  again  transformed 
into  motion. 

Hardly  a  tenth  of  the  electro-magnetic  energy  stored 
up  in  the  coal  is  given  out  by  the  steam  engine.  Our 
principal  source  of  electricity  to-day  is  the  steam  en- 
gine ;  yet  it  can  be  maintained  that  electricity  is  back 
of  the  steam  engine.  It  is  in  the  coal.  It  produced 
this  coal,  and  could  make  itself  manifest  to  a  large  de- 
gree if  we  knew  how  to  transform  it.  Many  attempts 
have  been  made  to  obtain  electrical  power  direct  from 
coal,  but  they  have  not  been  successful.  I  have  already 
referred  to  thermo-electricity.  We  have  seen  that  it  is 
possible  to  construct  a  furnace  with  junctions  of  suit- 
able metals  imbedded  in  its  fire  pot  so  that  the  com- 
bustion of  coal  will  produce  electrical  currents  in  these 


106  WHAT  IS  ELECTRICITY  f 

junctions.  Indeed,  an  electric  arc  light  has  been  formed 
by  such  means.  The  furnace,  however,  must  be  cum- 
brous, and  the  junctions  expand  and  contract  under  the 
changes  of  temperature  and  are  soon  ruined. 

We  have  seen  that  the  modern  dynamo  machine  is 
a  comparatively  simple  apparatus,  merely  a  number  of 
coils  on  a  revolving  shaft  surrounded  by  fixed  pieces  of 
iron,  around  which  the  currents  formed  in  the  revolving 
coils  are  made  to  circulate.  The  working  of  the  dynamo 
machine  is  entirely  dependent  upon  the  steam  engine 
which  drives  it.  In  the  best  dynamo  machine  only 
about  15  per  cent  of  the  energy  supplied  by  the  steam 
engine  is  lost.  When  we  consider  the  friction  of  the 
bearings  and  the  resistance  encountered,  the  economy 
of  the  transf ormation  of  energy  from  that  of  steam  to 
that  of  electricity  is  very  perfect.  The  modern  dynamo 
seems  to  have  reached  its  highest  development. 

The  spectacle  of  the  transformation  of  energy  by 
the  dynamo  in  our  great  cities  is  most  impressive.  At 
the  central  station  are  immense  steam  engines  which  are 
whirling  the  movable  coils  of  the  dynamos  on  axles 
which  run  at  about  one  thousand  revolutions  a  minute. 
The  earlier  dynamos  could  almost  be  lifted  by  one  man. 
The  dynamos  of  the  central  stations  weigh  tons.  Thus 
the  mechanical  engineer,  taking  the  principles  discov- 
ered by  Faraday,  has  adapted  it  to  a  very  perfect  ma- 
chine, and  has  made  one  of  the  greatest  transformations 
of  energy  witnessed  hi  the  mechanical  arts. 

Another  transformation  of  energy  resulted  from  the 
construction  of  the  dynamo  machine.  We  have  shown 
that  the  current  produced  by  one  dynamo  can,  if  led  to 
the  wires  of  a  second  dynamo,  make  the  movable  coils 
of  the  latter  revolve.  Thus  the  second  dynamo  becomes 
a  motor,  and  can  be  used  to  turn  shafting  or  set  in  mo- 


SOURCES  OF  ELECTRIC  POWER.  107 

tion  any  form  of  machinery.  Every  electric  car  has  a 
dynamo  motor  connected  with  its  axles,  and  the  current 
produced  by  the  great  dynamo  at  the  central  station 
sent  over  the  trolley  wire  propels  the  electric  cars. 
Now,  electro-magnetic  engines  were  not  unknown  in 
the  time  of  Faraday.  They  were,  however,  mere  lecture- 
table  models,  and  were  run  by  the  current  obtained 
from  batteries.  The  lecturer  of  twenty  years  ago  often 
took  such  models  as  a  text  to  show  the  impossibility  of 
obtaining  power  in  this  way,  for  a  short  calculation  of 
the  amount  of  zinc  consumed  in  the  battery  compared 
with  the  power  produced  showed  the  want  of  economy 
in  such  electric  motors.  Moreover,  the  mechanical  con- 
struction of  the  early  electro-motors  was  very  defective. 
The  idea  of  reciprocating  motion,  like  the  piston  of  a 
steam  engine,  was  the  ruling  one.  So  machines  were 
made  with  iron  cores  suspended  from  a  species  of  walk- 
ing beam,  like  that  of  a  steamboat.  These  cores  were 
alternately  sucked  into  or  repelled  from  coils  of  wire 
through  which  currents  of  electricity  were  circulating. 
The  early  investigators  were  appalled  by  the  quickness 
with  which  the  magnetic  forces  of  attraction  and  repul- 
sion decreased  with  the  distance.  These  forces  are  pro- 
portional to  the  product  of  the  attracting  masses  and  in- 
versely as  the  square  of  the  distance.  Thus  the  force  at 
the  distance  of  one  inch  is  only  one  fourth  of  that  at  one 
half  an  inch.  The  play  of  the  reciprocating  magnetic 
engine  was  therefore  very  small,  and  the  cost  of  the  zinc 
which  was  consumed  in  producing  this  small  play  was 
great.  There  seemed,  therefore,  little  hope  in  the  em- 
ployment of  magnetic  motors.  "With  the  discovery  of 
magnetic  induction  and  with  clearer  ideas  of  the  mag- 
netic field  surrounding  magnets  and  electro-magnets 
hopes  in  this  form  of  motor  revived  and  were  realized. 


108  WHAT  IS  ELECTRICITY  t 

Currents  of  electricity  could  be  produced  of  almost  un- 
limited strength  by  the  dynamo.  Yery  powerful  at- 
tracting magnets  could  therefore  be  made.  The  re- 
volving coils  of  the  dynamo  cut  the  lines  of  force  of  the 
stationary  electro -magnets  very  close  to  the  face  of  the 
poles  of  these  magnets,  where  the  number  of  lines  of  mag- 
netic force  are  greatest.  The  distance  between  the  re- 
volving coils  and  these  poles  in  some  cases  is  less  than 
one  eighth  of  an  inch.  When,  therefore,  an  electric 
current  is  sent  through  a  dynamo,  the  force  of  attraction 
between  the  revolving  coils  and  the  stationary  coils  is 
very  great,  since  the  distance  between  them  is  so  small ; 
and  the  commutator  changes,  as  I  have  said,  the  poles, 
so  that  a  continuous  rotation  of  the  movable  coils  is 
produced. 

The  mechanical  arrangement  which  we  have  called 
the  commutator  is  an  important  factor  in  the  produc- 
tion of  a  magnetic  motor.  A  clearer  conception,  how- 
ever, of  the  way  magnetic  lines  of  force  spread  out 
from  magnetic  poles  has  led  to  the  perfection  of  the 
magnetic  motor.  Every  one  is  familiar  with  the  way 
iron  filings  arrange  themselves  near  the  pole  of  a  mag- 
net. They  form  radiating  lines  which  spread  out  from 
the  poles  as  centres  of  disturbance,  and  seem  to  arch 
from  the  negative  to  the  positive  pole,  forming  what 
may  be  termed  magnetic  circuits  between  the  poles. 
If  our  magnet  is  made  in  the  form  of  a  horseshoe,  the 
arching  is  more  pronounced;  the  filings  crowd  into 
the  space  between  the  two  poles,  and  fewer  extend  into 
space.  By  making  the  horseshoe  nearer  and  nearer  the 
form  of  a  ring,  and  bringing  the  north  pole  almost 
into  contact  with  the  south  pole,  the  air  gap  between 
them  being  very  small,  hardly  any  lines  of  magnetic 
force  extend  into  the  space  about  the  poles.  They  are 


SOURCES  OP  ELECTRIC  POWER.  109 

concentrated  in  the  air  gap.  They  form  almost  a  closed 
magnetic  circuit  through  the  ring,  and  the  magnetic 
field  in  the  air  gap  can  thus  be  made  very  intense  by 
the  number  of  lines  of  magnetic  force  which  crowd 
into  this  space.  In  the  modern  dynamo  and  the  elec- 
tric motor  the  stationary  electro-magnets  are  made  as 
near  the  ring  form  as  possible.  The  revolving  coils 
are  placed  in  the  air  gap,  where  the  magnetic  lines 
crowd  from  one  pole  of  the  stationary  electro-magnet 
to  the  other.  Very  few  lines  of  force  are  suffered  to 
stray  out  of  the  air  gap  into  outer  space.  They  are 
nearly  all  cut  by  the  revolving  coils.  In  considering 
this  form  of  construction,  we  readily  see  how  imperfect 
the  early  electric  motors  were.  Very  few  of  the  lines 
of  magnetic  force  were  utilized  in  attraction.  Most  of 
them  strayed  into  the  air  and  were  lost,  so  far  as  useful 
work  was  concerned. 

With  the  perfection  of  the  electric  motor  the  steam 
engine  became  a  more  important  factor  in  civilization 
than  ever.  Electricity  appeared  to  be  ready  to  usurp 
its  place.  It  could  not  be  produced,  however,  econom- 
ically without  steam.  The  servant  could  not  take  the 
place  of  the  master.  A  new  method  of  distribution  of 
power,  however,  has  sprung  into  existence,  for  the 
electric  current  generated  by  steam  can  be  carried 
miles  from  the  producing  station,  and  can  be  trans- 
formed again  into  motion.  The  present  limit  of  dis- 
tance to  which  steady  currents  of  electricity  can  be 
economically  carried  for  conversion  into  power  is  about 
five  miles,  for  the  electricity  tends  to  escape  from  the 
wires  to  the  earth.  Moreover,  the  resistance  offered  to 
its  flow  by  the  wires  becomes  too  great.  I  have  said 
steady  currents ;  we  shall  see  later  that  the  distance  to 
which  fluctuating  currents  can  be  carried  is  far  greater 


HO  WHAT  IS  ELECTRICITY  I 

than  five  miles.  In  certain  manufacturing  establish- 
ments power  is  carried  from  one  building  to  another 
by  belts  and  shaftings  or  by  means  of  ropes.  The  loss 
in  transmission  on  account  of  friction  and  rigidity  of 
ropes  is  very  great  if  the  distance  exceeds  a  few  hun- 
dred feet.  It  is  estimated  that  in  transmitting  power 
by  means  of  electricity  five  miles  at  least  fifty  per  cent 
of  the  available  energy  is  lost.  The  only  ways,  how- 
ever, that  power  can  be  transmitted  five  miles  from  a 
central  station  are  by  means  of  a  current  of  electricity 
led  on  wires ;  by  means  of  storage  batteries ;  by  cables ; 
or  by  compressed  air.  In  all  of  these  methods  the 
steam  engine  is  the  source  of  the  energy.  The  elec- 
trical method  is  the  most  flexible  one,  for  wires  can  be 
easily  led  in  almost  any  direction.  In  the  great  cities 
at  present  the  electrical  energy  which  is  used  to  light 
the  city  at  night  is  employed  in  the  daytime  in  run- 
ning the  machinery  of  hundreds  of  workshops. 

In  the  many  examples  which  we  thus  see  of  the 
transformations  of  energy  accomplished  by  the  inven- 
tion of  the  dynamo  we  find  the  great  truth  of  the  con- 
servation of  energy  constantly  exemplified.  The  elec- 
trical engineer  calculates  the  efficiency  of  his  dynamo 
by  measuring  the  amount  of  work  the  steam  engine 
does  in  turning  the  swiftly  revolving  coil  of  the  dy- 
namo and  comparing  this  with  the  amount  of  work  the 
electricity  can  do.  He  assumes  that  if  the  mechanism 
of  conversion  were  perfect  the  energy  produced  must 
be  equal  to  that  expended.  Tyndall,  in  his  Heat  as  a 
Mode  of  Motion,  showed  that  friction  could  be  con- 
verted into  heat  by  rapidly  revolving  a  closed  tube  con- 
taining water  between  a  suitable  clamp.  By  means  of 
dynamos  and  motors  I  have  modified  this  experiment 
in  the  following  manner:  Let  a  steam  engine  set  in 


SOURCES  OP  ELECTRIC  POWER.       m 

action  a  dynamo ;  then  the  dynamo  an  electric  motor. 
On  the  continuation  of  the  shaft  of  the  latter  place  a 
hollow  tube  partially  filled  with  water  and  tightly 
corked.  If  now  a  friction  brake  clasps  this  tube  and 
the  motor  is  set  in  revolution,  hi  a  few  seconds  the 
water  boils,  steam  is  formed,  and  the  cork  is  thrown 
across  the  room  by  the  expanding  steam.  Thus  the 
steam,  through  various  transformations,  produces  again 
steam,  and  the  work  done  against  the  friction  brake 
can  be  estimated  by  the  amount  of  water  raised  to  212° 
F.  This  in  modern  form  is  the  celebrated  experiment 
of  Count  Kumf  ord  by  means  of  which  he  set  the  world 
to  thinking  about  the  conversion  of  work  into  heat. 

We  have  followed  the  work  of  steam  from  the  en- 
gine to  the  dynamo,  where  its  energy  is  converted  into 
electrical  energy,  from  the  reconversion  of  this  energy 
by  means  of  the  electric  motor  to  motion,  and  we  have 
seen  that  steam  is  still  the  great  moving  power  in  the 
machinery  of  the  world,  while  electricity  can  be  said  to 
be  only  its  servant,  nimble  and  pliable.  "We  have  said 
nothing  of  the  transformation  of  water  power  into  elec- 
tricity. "Why  is  not  this  source  of  power  cheaper  than 
steam  to  produce  electricity  ?  The  modern  stationary 
engine  has  been  brought  to  such  a  state  of  perfection 
that  a  horse  power  can  be  obtained  from  a  pound  and 
two  tenths  of  coal,  while  the  cost  of  regulating  a  water 
supply  costs  more  than  this.  At  present  electricity  can 
be  manufactured  in  Buffalo  from  coal  cheaper  than  it 
can  be  transmitted  from  Niagara  Falls  to  Buffalo,  a 
distance  of  about  thirty  miles.  We  shall  return  to 
the  subject  of  the  electrical  transmission  of  power  in 
a  subsequent  chapter.  Now  we  are  considering  the 
transformation  of  energy  by  means  of  steady  currents 
of  electricity.  Long-distance  transmission  of  power 


112  WHAT  IS  ELECTRICITY  1 

can  not  be  accomplished  by  steady  currents  unless  some 
efficient  form  of  storage  battery  should  be  invented. 
The  storage  battery  illustrates  another  transformation 
of  energy  which  awakened  great  hopes,  but  which  has 
not  yet  fulfilled  the  expectations  of  mankind.  The 
storage  battery  or  accumulator  in  its  commonest  form, 
we  have  seen,  consists,  before  it  is  charged,  of  red  oxide 
of  lead  with  electrodes  of  lead.  The  oxide  and  the 
electrodes  are  suitably  immersed  in  dilute  sulphuric 
acid,  and  a  strong  current  of  electricity  is  sent  from 
one  electrode  to  the  other.  The  oxygen  resulting  from 
the  electrolysis  of  the  water  converts  the  red  oxide  of 
lead  at  the  future  positive  pole  of  the  battery  into  per- 
oxide of  lead  and  into  metallic  lead  at  the  future  nega- 
tive pole  of  the  cell.  When  the  charging  current  is 
removed  and  the  peroxide-of-lead  pole  is  connected 
with  the  porous  metallic  lead  pole,  a  current  of  elec- 
tricity is  produced  and  the  peroxide  goes  back  to  a  lower 
oxide.  The  transformation  from  the  energy  of  steam 
to  that  of  electricity  in  this  case  is  by  means  of  chem- 
ical action,  and  chemical  action  has  not  yet  produced 
electricity  economically. 

I  suppose  that  no  subject  in  the  development  of 
electricity  has  received  so  much  attention  as  that  of 
storage  batteries.  It  was  quickly  shown  that  the  elec- 
tro-magnetic engines  run  by  batteries  were  far  more 
expensive  to  run  than  the  steam  engine ;  for  in  order 
to  smelt  the  zinc  for  the  plates  of  a  battery  it  was  ne- 
cessary to  expend  about  sixty  times  its  own  weight  in 
coal,  while  a  pound  of  coal  could  produce  a  much 
greater  amount  of  horse  power  than  a  pound  of  zinc. 
If  we  should  supplant  the  steam  engine  at  a  central 
station  by  a  great  battery  plant  we  should  consume  zinc 
in  sulphuric  acid  instead  of  coal  under  a  boiler.  The 


SOURCES  OP  ELECTRIC  POWER.  H3 

consumption  of  zinc  in  sulphuric  acid  generates  energy 
in  the  form  of  heat.  By  connecting  the  poles  of  a  bat- 
tery with  a  copper  wire  we  convert  this  energy  into 
electricity,  and  reconvert  it  into  heat  wherever  the  re- 
sistance of  the  line  is  sufficient.  The  transformation  of 
energy  thus  proceeds  from  chemical  action  through 
electrical  action  back  to  heat.  Batteries  in  which  chem- 
ical action  is  used  to  consume  zinc  are  called  primary 
batteries,  and  inventors  are  still  busy  in  endeavouring 
to  produce  an  economical  chemical  source  of  electricity, 
but  none  has  yet  been  discovered.  Indeed,  the  use 
of  primary  batteries  is  daily  growing  more  limited,  and 
the  dynamo  machine  and  storage  batteries  are  taking 
their  place.  A  storage  battery  can  be  made  which  will 
give  back  nearly  eighty  per  cent  of  the  energy  which 
is  used  to  charge  it.  Its  life,  however,  is  limited.  It 
is  fully  as  long,  however,  as  that  of  the  best  primary 
battery. 

While  the  storage  battery  has  not  fulfilled  the  hopes 
it  excited,  it  has  gradually  come  into  commercial  use 
in  steadying  the  load  at  central  power  and  light  stations. 
In  case  of  an  accident  to  the  machinery  the  current 
from  a  storage  battery  can  be  turned  on  the  line.  It 
acts  like  a  spare  reservoir  of  water  for  sudden  emer- 
gencies. 

It  is  probable  that  some  other  substance  better  fitted 
than  lead  for  the  use  of  accumulators  will  be  discovered. 
One  can  to-day  use  peroxide  of  lead  and  zinc  in  a 
storage  battery  and  obtain  more  powerful  currents  than 
with  the  use  of  lead  for  both  elements  of  a  storage  cell. 
This  is  a  suggestion  of  De  la  Rive,  and  it  has  been 
developed  by  Prof.  Main  into  a  zinc-lead  storage 
battery.  The  zinc,  however,  consumes  rapidly,  and 
there  are  chemical  reactions  which  are  troublesome  and 


114  WHAT  IS  ELECTRICITY? 

destructive.  I  have  found  the  following  method,  how- 
ever, of  transforming  energy  by  means  of  zinc  ex- 
tremely useful  in  the  laboratory.  In  order  to  under- 
stand it  one  must  look  at  an  old  form  of  primary  battery 
which  even  now  is  used  in  many  laboratories  where  a 
portable  source  of  electricity  is  required.  It  is  called 
the  dip  battery,  and  consists  of  alternate  plates  of  car- 
bon and  zinc,  which  are  immersed,  when  used,  in  a  solu- 
tion of  bichromate  of  potassium.  The  carbon  consti- 
tutes the  positive  pole  and  the  zinc  the  negative.  The 
zinc  is  consumed  by  chemical  action,  and  a  current  of 
electricity  is  produced  by  this  energy  along  the  wire 
connecting  the  carbon  pole  with  the  negative.  When 
the  battery  is  not  in  use  the  plates  are  lifted  out  of  the 
bichromate  of  potassium.  The  chemical  action  is  of 
short  duration,  and  an  hour's  use  generally  depletes  the 
battery.  Instead  of  carbon  I  have  used  porous  parti- 
tions filled  with  oxide  of  lead  converted  into  peroxide 
of  lead.  In  charging,  the  negative  electrode  is  a  simple 
lead  plate.  After  the  cell  is  charged  the  negative  lead 
plates  are  lifted  from  the  sulphuric  acid  and  amalga- 
mated zinc  plates  are  lowered  in  their  place.  We  have 
thus  a  modern  dip  battery  immensely  more  powerful 
and  serviceable  than  the  old  form  of  primary  dip  bat- 
tery. This  use  of  zinc  led  me  to  an  interesting  method 
of  regenerating  an  old  used-up  storage  cell  which  was 
incapable  of  storing  up  the  energy  supplied  to  it.  The 
use  of  a  zinc  electrode  restores  the  positive  lead  plate 
to  usefulness  by  breaking  up  an  injurious  sulphate  of 
lead  which  is  formed.  Two  recent  observers,  MM.  Cail- 
letet  and  Coladeau,  have  succeeded  in  storing  in  the 
metal  palladium  under  six  hundred  atmospheres  pres- 
sure more  than  ten  times  the  amount  of  electric  energy 
which  can  be  stored  in  the  same  weight  of  lead  oxide. 


SOURCES  OP  ELECTRIC  POWER.       ^5 

The  method,  however,  of  transforming  energy  by  means 
of  electrolysis  does  not  at  present  seem  to  be  econom- 
ical. 

There  is  another  method  of  obtaining  electricity 
from  coal  without  the  formation  of  steam,  and  this 
consists  in  the  employment  of  gas  engines.  Ordinary 
illuminating  gas  mixed  with  the  proper  proportion  of 
air  is  exploded  in  a  cylinder,  and  the  explosion  drives  a 
piston  to  and  fro.  Certain  writers  contend  that  it  is 
cheaper  to  drive  a  dynamo  by  such  an  engine  and  pro- 
duce light  by  electricity  than  to  burn  the  gas  direct. 
Great  improvements  have  been  made  in  this  method  of 
producing  electricity.  Gasoline,  for  instance,  can  be 
used  instead  of  illuminating  gas.  "With  the  gas  engine 
all  the  transformations  of  energy  which  we  have  shown 
can  be  accomplished  by  steam  are  possible.  We  have 
thus  another  possible  method  of  the  transmission  of 
power.  Gas  can  be  distributed  from  a  central  station 
instead  of  electricity,  and  can  be  converted  into  elec- 
tricity when  the  latter  is  wanted.  A  gas  engine  does 
not  require  the  services  of  a  skilful  engineer  and  of  a 
fireman.  Merely  lighting  the  gas  torch  in  the  engine 
with  a  match  sets  the  engine  in  motion.  The  tendency 
of  the  times  is  to  use  more  concentrated  fuel  than  coal. 
Thus  we  have  oil  engines  and  naphtha  launches.  Maxim 
guns  may  also  be  considered  a  species  of  gas  engine. 
This  powerful  weapon  is  capable  of  discharging  six 
hundred  shots  a  minute.  The  recoil  from  the  first 
shot  brings  another  cartridge  into  position  where  it  is 
fired,  and  so  on.  The  gun  is  self-acting  after  the  first 
shot.  With  the  powerful  explosives  at  our  command 
the  power  we  could  exert  in  engines  is  enormous.  The 
difficulty  is  in  properly  controlling  and  employing  the 
energy  of  the  explosion. 


116  WHAT  IS  ELECTRICITY? 

The  subject  of  gas  engines  brings  us  to  a  considera- 
tion of  compressed-air  motors.  Many  believe  that  the 
employment  of  compressed  air  as  a  source  of  power  on 
street  railways  has  not  been  sufficiently  tested.  The 
modern  mechanical  engineer  has  succeeded  in  making 
cylinders  of  steel  which  can  stand  with  safety  the  enor- 
mous pressure  of  60,000  pounds  to  a  square  inch,  and 
it  does  not  seem  impossible  that  we  may  yet  see  com- 
pressed-air motors  actuating  machinery  which  is  now 
driven  by  electricity.  In  the  Calumet  and  Hecla  cop- 
per mine  power  is  supplied  in  the  mine  by  compressed 
air.  The  loss  at  the  distance  of  three  miles  is  about 
fifty  per  cent,  while  the  loss  by  electricity  is  not  far 
from  the  same  amount.  Greater  safety  results  from 
the  use  of  compressed  air  in  mines  than -from  the  em- 
ployment of  electricity,  for  there  is  no  danger  from  fire 
by  this  method  of  transmission  of  power. 

Maxim,  in  his  interesting  experiments  on  aerial  navi- 
gation, has  entered  carefully  into  an  estimate  of  the 
power  that  can  be  developed  by  various  kinds  of  motors 
in  comparison  with  their  weight,  and  gives  the  follow- 
ing :  Hot-air  engines,  200  pounds  to  the  horse  power ; 
Brayton's  oil  engine,  75  pounds  to  the  horse  power ; 
electric  motors  fed  by  secondary  batteries,  130  pounds 
to  the  horse  power ;  gas  engines  (Otto),  50  pounds  to 
the  horse  power;  steam  engines,  with  condenser, 
pumps,  and  everything  complete,  25  to  50  pounds  to 
the  horse  power. 

Maxim's  complete  steam  motor  as  it  existed  in  1894 
weighs  2,040  pounds,  which  includes  the  boiler,  engines, 
gas  generator,  pumps,  and  200  pounds  of  water  in  the 
boiler,  but  this  does  not  include  the  supply  of  fuel,  the 
water  in  the  tank,  or  the  condenser.  The  highest  power 
developed  was  363  horse  power,  which  gives  5'6  pounds 


SOURCES  OP  ELECTRIC  POWER.  H7 

to  the  horse  power.  An  atmospheric  condenser  can 
be  made  weighing  no  more  than  one  pound  to  the 
horse  power. 

The  transformation  of  enhanced  molecular  activity 
through  the  varied  chain  of  motion  of  machinery, 
whirls  of  magnetic  lines,  to  the  storage  battery,  where 
the  molecular  action  again  reappears,  is  not  more  strik- 
ing than  the  transformations  which  can  be  accomplished 
by  the  battery.  Thus  machinery  can  be  turned,  mag- 
netic fields  formed.  In  both  transformations  incan- 
descent lamps  can  be  lighted  and  water  converted  into 
Bteam.  Still  further,  the  electric  light  generated  by 
the  molecular  activity  of  the  steam  or  the  molecular 
activity  of  the  storage  battery  can  set  up  or  modify  the 
molecular  activity  of  a  sensitive  photographic  plate, 
producing  before  accomplishing  this  result  waves  in  the 
ether  of  space.  It  is  probable  that  we  know  little  of  the 
effect  of  electro-magnetic  waves  in  the  ether  in  trans- 
forming or  changing  the  rates  of  molecular  motion. 
Attempts  are  being  made  to  ascertain  the  effect  of  elec- 
trolysis on  plant  growth.  Certain  portions  of  planted 
fields  in  France  have  been  submitted  to  the  effect  of  the 
electric  current — they  have  been  charged,  so  to  speak, 
like  storage  batteries — while  other  portions  have  not 
been  thus  treated.  It  is  said  that  a  difference  in  the  rate 
of  germination  and  growth  of  plants  has  been  detected. 
The  electrical  treatment  apparently  has  stimulated  the 
vegetation.  Siemens  submitted  certain  plants  to  the 
rays  of  the  electric  light  for  a  long  period  of  time,  and 
greatly  enhanced  their  growth.  The  molecular  action 
in  one  case  was  stimulated  at  the  root  and  in  the  other 
in  the  leaves,  and  the  ultimate  source  of  the  stimulus 
was  in  the  coal  measures,  produced  by  a  luxuriant  vege- 
tation of  ages  ago,  which  in  turn,  according  to  modern 


118  WHAT  IS  ELECTRICITY? 

belief,  had  its  source  of  growth,  in  electro-magnetic 
waves  from  the  sun. 

In  our  account  of  the  transformations  of  motion  into 
electricity  we  have  dwelt  largely  on  the  conversion  of 
electricity  again  into  motion.  The  production  of  light 
by  electricity  has  become  a  matter  of  such  daily  obser- 
vation that  we  have  ceased  to  wonder  at  it.  Attempts 
have  also  been  made  to  heat  economically  by  electricity. 
All  that  is  necessary  is  to  increase  the  resistance  of  the 
electrical  circuit  at  the  point  where  we  desire  to  pro- 
duce the  heat. 

We  have  followed  the  transformations  which  result 
from  burning  coal,  the  fossilized  vegetation  of  a  former 
age ;  let  us  now  examine  the  transformations  exerted 
by  man  considered  as  an  engine.  The  food  he  con- 
sumes answers  to  the  fuel  we  put  under  the  boiler  of  the 
steam  engine.  The  source  of  this  food  is  also  vegeta- 
tion, and  the  ultimate  source  of  vegetation  is,  according 
to  modern  theories,  the  electro-magnetic  radiations  of 
the  sun.  Animals  and  man  take  in  oxygen  and  give 
out  carbonic  acid.  It  was  pointed  out  by  Joule  that 
man  more  nearly  resembles  an  electro-magnetic  engine 
than  a  steam  engine,  and  he  also  showed  that  man  as  an 
engine  is  far  more  efficient  than  any  form  of  engine 
which  he  can  construct  in  which  there  is  a  consumption 
of  fuel.  Man  may  be  roughly  compared  to  a  voltaic 
cell.  In  the  cell  we  have  sulphuric  acid  and  zinc  in  the 
shape  of  food,  while  the  resulting  energy  in  the  form  of 
electricity  can  be  transformed  into  motion  or  heat.  In 
man  food,  together  with  oxidizing  processes,  produce 
also  motion  and  animal  heat.  Yain  attempts  have  been 
made  to  measure  the  efficiency  of  the  man  engine  by 
weighing  the  food  consumed  and  measuring  the  work 
done.  The  transformations  of  the  food,  however,  are 


SOURCES   OP  ELECTRIC  POWER.  H9 

so  varied  and  so  subtile  that  it  is  difficult  to  estimate 
them.  In  general,  however,  the  amount  of  work  a  man 
does  bears  a  certain  proportion  to  the  fuel  he  puts  into 
his  boiler.  It  is  interesting  to  notice  that  in  the  mod- 
ern practice  of  medicine  the  conservation  of  energy  is 
recognised.  Heat  is  supplied  to  invalids  in  order  that 
the  human  engine  may  not  be  compelled  to  supply  this 
animal  heat.  Too  great  an  output  of  heat  in  the  case 
of  fevers  is  checked  by  the  application  of  cold  water. 
Yarious  instruments,  similar  to  indicators  used  to  ascer- 
tain the  horse  power  of  steam  engines,  are  employed  by 
physiologists  to  study  the  action  of  the  heart  considered 
as  a  pumping  engine.  Perhaps  the  French  physiolo- 
gist Marey  has  more  than  any  one  else  called  attention 
to  the  importance  of  studying  the  action  of  different 
parts  of  the  human  engine. 

Now  the  plants  also  constitute  forms  of  engines 
which  use  the  products  rejected  by  men  and  animals. 
By  means  of  the  radiant  energy  of  the  sun  they  are 
enabled  to  decompose  carbonic  acid.  One  can  compare 
a  plant  to  a  storage  cell  in  which  a  current  of  electricity 
decomposes  the  liquid  into  oxygen  and  hydrogen,  and 
forms  materials  which  again  give  electricity  and  mo- 
tion and  heat.  The  sun  enables  the  plant  engine  to 
work.  The  rays — so-called  chemical  rays — that  is,  the 
shortest  waves  of  light,  are  most  effective  in  producing 
the  decomposition  of  carbonic  acid  by  the  leaves  of 
plants.  These  rays  are  those  which  are  the  most  ab- 
sorbed by  the  foliage  of  plants.  This  is  shown  by  the 
ordinary  photograph  of  a  landscape.  The  leaves  are 
black  except  when  they  reflect  light.  They  have  ab- 
sorbed the  rays  of  light,  and  have  decomposed  carbonic 
acid  and  water.  The  efficiency  of  the  plant  engine  is 
still  more  difficult  to  obtain  than  that  of  the  man  engine, 


120  WHAT  IS  ELECTRICITY? 

for  the  subtile  transformations  of  the  chlorophyll  are 
many.  We  have  the  innumerable  coal-tar  dyes,  which 
seem  to  have  as  many  peculiar  properties  as  they  have 
colours.  Also  in  photography  we  have  hydroquinone, 
rodinal,  amidol,  and  a  host  of  other  developers  which 
are  used  to  give  pictures  of  the  very  plants  which  by 
previous  transformation  have  produced  them. 


CHAPTEE  X. 

TRANSFORMATIONS    OF   ENEEGY. 

I  HAVE  dwelt  upon  the  construction  of  a  galvanome- 
ter and  of  the  dynamo  machine  in  order  to  emphasize 
the  great  bearing  that  properly  constructed  machines 
have  upon  the  progress  of  science.  Faraday's  galva- 
nometer was  not  a  very  sensitive  instrument ;  it  was  anal- 
ogous to  the  one-lens  microscope  of  Leuwenhoek ;  yet  it 
was  sufficient  to  show  the  great  law  of  magneto-induc- 
tion, that  the  movement  of  a  coil  of  wire  near  a  mag- 
netic pole  produces  a  current  of  electricity  in  the  coil, 
and  when  it  was  aided  by  powerful  auxiliaries  it  showed 
that  any  change  in  an  electric  current  produced  a  cur- 
rent of  electricity  through  the  ether  of  space  in  neigh- 
bouring conductors.  These  powerful  helps  consisted  of 
a  strong  voltaic  battery  of  large  plates,  and  a  great  many 
of  them,  and  of  a  powerful  electro-magnet  which  Joseph 
Henry  had  shown  how  to  construct  and  how  to  use  with 
the  battery  in  order  to  obtain  the  greatest  effect.  Fara- 
day made  his  principal  discoveries  in  magneto-induction 
in  ten  days  when  he  was  at  the  age  of  forty-two.  There 
is  little  doubt  in  my  mind  that  other  men  would  speed- 
ily have  discovered  the  same  phenomena,  for  their  uni- 
versality could  not  long  have  eluded  observation.  Fara- 
day's great  achievement  was  in  his  conception  of  the 


122  WHAT  IS  ELECTRICITY? 

lines  of  force  which  emanate  from  a  magnetic  pole  and 
stretch  through  the  ether  of  space ;  in  his  pointing  out 
that  the  medium  surrounding  the  wires  carrying  electric 
currents,  and  the  medium  in  which  magnets  are  situ- 
ated, is  in  a  state  of  strain ;  that  there  is  what  he  called 
an  electrotonic  state  of  this  medium.  It  was  like  a  mass 
of  quivering  jelly — any  movement  at  one  point  pro- 
duced a  quiver  in  all  neighbouring  points. 

Maxwell,  in  his  great  work  on  electricity,  thus 
speaks  of  Faraday's  conception :  "  Faraday  saw  lines  of 
force  traversing  all  space  where  the  mathematicians  saw 
centres  of  force  attracting  at  a  distance.  Faraday  sought 
the  seat  of  the  phenomena  in  real  actions  going  on  in 
the  medium ;  they  were  satisfied  that  they  had  found  it 
in  a  power  of  action  at  a  distance  impressed  on  the  elec- 
tric fluids."  *  One  obtains  a  realizing  sense  of  the  in- 
tellectual power  of  Faraday  by  reading  his  Experimental 
[Researches  in  Electricity.  He  is  not  merely  an  inventor 
who,  having  discovered  some  phenomenon  of  Nature, 
proceeds  to  put  it  to  a  practical  use ;  but  each  experi- 
ment is  guided  by  a  remarkable  generalizing  faculty, 
and  the  series  of  his  experimental  researches  led  him  to 
the  conception  of  electrical  actions  in  the  medium  of 
space  and  laid  the  foundation  of  the  greatest  general- 
ization in  science  of  modern  times — Maxwell's  electro- 
magnetic theory  of  light.  We  shall  be  led  to  this  great 
theory  as  we  continue  our  study  of  the  question,  "  What 
is  electricity  ? "  Meanwhile  let  us  examine  a  little  fur- 
ther the  phenomena  discovered  by  Faraday. 

A  delicate  galvanometer  reveals,  we  have  seen,  that 
the  motion  of  a  wire  near  a  magnet  results  in  an  elec- 
tric current  in  the  wire.  Why  this  is  so  we  do  not 

*  Preface  to  Maxwell's  Electricity  and  Magnetism. 


TRANSFORMATIONS  OP  ENERGY.  123 

know.  We  have  a  good  working  theory,  however, 
which  we  shall  express  later. 

Since  the  earth  is  a  magnet,  any  motion  of  a  wire 
on  its  surface  will  cause  a  current  in  the  wire.  One 
can  signal  under  the  sea  through  a  cable  by  properly 
waving  a  coil  of  wire  in  the  air.  We  say  that  the  re- 
sulting current  of  electricity  is  due  to  cutting  the  lines 
of  magnetic  force  of  the  earth  by  the  motion  of  the 
wire. 

For  forty  years,  nearly  half  a  century  after  Fara- 
day's great  discovery  of  magneto-induction,  men's  minds 
were  almost  exclusively  devoted  to  obtaining  steady 
currents  of  electricity  in  one  direction  instead  of 
to-and-fro  currents,  such  as  are  obtained  by  rapidly 
thrusting  a  north  pole  into  a  spool  of  wire  and  rapidly 
withdrawing  it.  The  commutator  was  unproved  in 
every  possible  way  until  the  commuting  of  the  direc- 
tions of  the  to-and-fro  currents  had  well-nigh  become 
perfect.  The  commutator  hi  the  best  forms  of  the 
modern  dynamo  machine  shows  very  little  sparking, 
whereas  in  the  earlier  forms  there  was  a  brilliant  cor- 
uscation of  sparks  when  the  segments  of  the  commu- 
tator ran  under  the  brushes  which  collected  the  current 
for  the  outer  circuit  hi  which  the  electrical  work  was 
to  be  done.  These  sparks  showed  that  energy  was  lost. 
There  is  very  little  room  for  improvement  at  present  in 
the  modern  dynamo.  It  approaches  nearer  to  perfec- 
tion than  any  other  machine  which  is  used  to  transmit 
power.  It  is  doubtful  in  my  mind  whether  Faraday 
ever  realized  the  powerful  effects  that  could  be  obtained 
when  his  lines  of  magnetic  force  were  made  to  quiver 
with  great  speed.  The  galvanometer  employed  by  him 
could  only  detect  steady  currents,  or  momentary  cur- 
rents which  reversed  in  direction  very  slowly.  It  was 


124  WHAT  IS  ELECTRICITY? 

incapable  of  showing  any  effect  when  the  currents  of 
induction  were  sent  to  and  fro  through  it  very  rapidly. 
It  would  remain  perfectly  quiescent,  its  little  needles 
pointing  placidly  north  and  south,  while  to-and-f  ro  cur- 
rents of  tremendous  energy  were  circulating  through 
the  circuit  with  which  it  was  connected.  It  was  like  a 
deaf-mute  with  respect  to  the  world  of  harmonies  of  an 
orchestra. 

Although  these  to-and-fro  currents  of  electricity 
annulled  each  other's  effect  on  the  needle  of  the  gal- 
vanometer, they  could  produce  an  electric  light,  could 
heat  wires,  and,  in  short,  produce  all  the  effects  obtained 
from  steady  currents  with  the  exception  that  they  could 
not  run  an  electric  motor  of  the  type  which  we  have 
considered — the  type  which  had  been  slowly  perfected 
during  the  forty  years  after  Faraday's  discoveries.  We 
are  now  entering  upon  another  period  of  electrical  in- 
vention which  may  be  called  the  period  of  adaptation 
of  Faraday's  discoveries  to  instruments  adapted  to  to- 
and-fro  currents  instead  of  steady  currents,  and  we  shall 
see  the  necessity  of  using  to-and-fro  currents  instead  of 
steady  currents  for  the  transmission  of  electrical  power, 
as  we  continue  our  study.  I  have  said  that  it  is  ques- 
tionable in  my  mind  whether  Faraday  realized  the 
wonderful  development  which  is  now  beginning  in  the 
commercial  employment  of  to-and-fro  currents.  He 
certainly,  however,  had  a  full  conception  of  the  sensi- 
tiveness of  the  electrotonic  state  of  the  medium  sur- 
rounding magnets  and  electrical  circuits,  but  he  had  no 
instruments  which  could  represent  to  other  people's 
eyes  and  ears  the  wonders  of  his  imagination. 

The  telephone  is  an  instrument  based  entirely  upon 
Faraday's  discovery  of  magneto-induction.  If  we 
should  take  the  spools  which  we  have  used  (Fig.  10)  to 


TRANSFORMATIONS  OF  ENERGY. 


125 


illustrate  the  action  of  a  commutator,  slip  each  of  them 
upon  the  poles  of  a  magnet,  as  in  Fig.  12,  connect  the 
ends  of  the  wires  of  the  spools  permanently,  place  a 
thin  disk  of  iron,  such  as  is  used  in  taking  tintypes  in 
photography,  very  near  to  each  pole,  providing  a  suit- 
able earpiece  to  each  telephone,  we  would  find  that  a 
mere  tap  with  the  finger  on  the  iron  disk  of  tele- 
phone A,  for  instance,  can  be  heard,  on  listening  at 
that  of  telephone  B,  even  when  the  telephone  B  is 

A  B 


at  the  distance  of  many  miles  from  telephone  A. 
The  operation  of  this  tap  is  like  thrusting  rapidly  a 
magnetic  pole  into  the  spool  of  B  and  quickly  with- 
drawing it :  to-and-fro  currents  of  induction  are  pro- 
duced which  attract  and  repel  the  disk  of  the  telephone 
B,  thus  reproducing  the  mechanical  action  which  is  ex- 
erted by  tapping  the  instrument  A.  It  is  more  correct 
as  well  as  more  picturesque  to  say  that  the  quivering  of 
the  magnetic  lines  of  force  due  to  the  slight  movement 
of  the  thin  iron  disks  affects  the  electrotonic  state  of 
the  medium  in  which  the  telephones  are  immersed. 
The  telephone  into  which  we  speak  corresponds  to  the 
electric  dynamo,  and  the  telephone  to  which  we  listen 
to  the  electric  motor.  But  here  there  is  no  commutator ; 
we  are  using  to-and-fro  currents,  or,  in  ordinary  prac- 


126  WHAT  IS  ELECTRICITY? 

tical  language,  we  have  an  alternating  current  dynamo 
and  an  alternating  current  motor.  The  excursions  or 
movements  to  and  fro  of  the  telephone  diaphragm  are 
exceedingly  small,  and  can  only  be  detected  under  the 
microscope.  Indeed,  the  telephone  disk  provided  with 
a  suitable  pointer,  in  conjunction  with  a  microscope, 
has  been  suggested  as  a  suitable  alternating  current  gal- 
vanometer, and  Prof.  Rubens,  of  Berlin,  has  lately 
shown  how  a  practical  instrument  can  be  made  on  a 
similar  principle. 

If  now  we  cause  a  sufficiently  powerful  alternating  or 
to-and-fro  current  to  circulate  through  a  coil  of  wire 
wrapped  around  a  piece  of  iron,  or,  in  other  words,  if  we 
use  the  big  magnet  employed  by  Faraday  and  Henry  in 
their  researches,  we  find  that,  although  the  galvanom- 
eter is  absolutely  quiescent  and  gives  no  indication  of 
the  great  fluctuations  of  magnetism  in  the  magnet,  a 
telephone  connected  to  a  little  coil  of  wire  which  is 
placed  between  the  poles  of  the  electro-magnet  gives 
forth  a  loud  hum,  which  is  the  note  of  the  periodic  cur- 
rents ;  it  is  their  rate  of  alternation.  Furthermore,  by 
moving  the  little  coil  about  the  poles  of  the  magnet  we 
can  trace  the  spreading  of  the  magnetic  lines  into  the 
air,  and  thus  through  the  ear  obtain  the  same  concep- 
tion of  this  spreading  as  we  obtain  when  we  sprinkle 
iron  filings  on  a  piece  of  paper  and  place  the  paper  on 
the  poles  of  the  magnet.  Moreover,  it  is  not  necessary 
to  connect  the  telephone  with  a  little  coil  of  wire  ;  we 
can  even  remove  the  coil  from  the  interior  of  the  tele- 
phone, and  thus  with  nothing  but  a  thin  disk  of  iron 
placed  close  to  the  pole  of  a  magnet,  by  placing  this 
dissected  telephone,  so  to  speak,  to  the  ear,  we  can 
hear  the  hum  of  the  electro-magnet  in  all  parts  of  a 
large  room,  thirty  or  forty  feet  from  the  magnet,  and  by 


TRANSFORMATIONS  OF  ENERGY.  127 

walking  about  the  magnet  we  can  trace  the  paths  of  the 
lines  of  force  by  the  sense  of  hearing.  We  thus  per- 
ceive that  the  entire  space  of  the  room  is  filled  with  these 
quivering  lines  of  force.  To-and-fro  currents  thus  ap- 
peal to  another  sense — that  of  hearing — and  greatly  en- 
large our  conception  of  Faraday's  electrotonic  state. 
The  lines  of  magnetic  force  crowd  together  into  the  thin 
disk  and  the  magnet  wliich  form  our  exploring  instru- 
ment. They  prefer  to  pass  through  iron  or  steel  to 
passing  through  the  air.  Thus,  by  placing  a  bar  of  iron 
across  the  poles  of  the  electro -magnet  we  cease  to  hear 
the  hum  of  the  magnet  in  our  exploring  telephone. 
The  lines  of  force  prefer  to  pass  from  pole  to  pole 
through  the  bar  of  iron  to  spreading  out  into  the  room. 
Now,  if  we  could  fill  a  room  with  these  invisible 
lines  of  magnetic  force  of  sufficient  strength  we  could 
light  an  electric  lamp  in  every  part  of  it.  The  lamp 
would  light  when  we  entered  the  magnetic  field,  and  go 
out  when  we  retired  from  it.  No  matches  would  be  re- 
quired. We  can  realize  this  ideal  system  of  lighting 
already  in  a  small  way,  which  I  have  often  employed 
in  my  lectures  to  illustrate  lighting  up,  so  to  speak,  the 
magnetic  curves,  such  as  we  see  depicted  in  arrange- 
ments of  magnetic  filings.  Connect  a  small  coil  of  a 
suitable  number  of  windings  with  a  little  incandescent 
lamp,  such  as  is  now  used  for  lighting  houses,  but  much 
smaller,  move  this  coil  near  the  poles  of  an  electro- 
magnet through  which  a  powerful  alternating  current  is 
flowing — the  lamp  will  light  in  certain  positions  of  the 
coil  in  mid  air.  Moreover,  we  can  move  it  about  the 
poles  of  the  electro-magnet  so  as  to  illustrate  the  curv- 
ing of  the  lines  of  force.  In  order  to  fill  a  room  with 
suitably  strong  lines  of  magnetic  force  to  enable  us  to 
light  a  lamp  in  any  part  of  the  room  by  these  invisible 


128  WHAT  IS  ELECTRICITY? 

lines  of  force,  we  should  need  enormous  magnets  and 
enormously  strong  lines  of  force.  Niagara  Falls,  how- 
ever, is  competent  to-day  to  enable  us  to  realize  this 
flight  of  our  imagination.  It  is  interesting  to  observe 
that  we  could  move  about  in  this  great  field  of  magnetic 
energy  with  no  sense  of  discomfort  save  from  the  hum 
of  the  alternating  currents  in  the  electro-magnet.  There 
would  be  no  sense  of  strain  or  pressure  in  the  head.  If 
it  were  not  for  the  hum,  we  should  not  be  conscious  of 
the  tremendous  energy  in  the  space  of  the  room. 

But  I  hear  some  one  exclaim,  "  Are  not  certain  peo- 
ple sensitive  to  magnets  ? "  This  belief  has  been  held  at 
various  times  by  many  persons,  and  after  suitable  inter- 
ment it  rises  again  to  perplex  humanity.  On  this  sub- 
ject one  should  consult  Baron  Reichenbach's  treatise 
on  what  he  terms  the  odic  force  (see  page  40  of  this 
treatise). 

The  future  development  of  the  practical  employ- 
ment of  electricity  in  lighting  and  the  transmission 
of  power  must  be  in  the  direction  of  alternating  cur- 
rents instead  of  steady  currents.  We  shall  see  that  Na- 
ture sends  us,  so  to  speak,  our  electricity  by  means  of 
to-and-fro  movements  from  the  sun ;  and  in  order  to 
approximate  to  the  economy  of  Nature,  we  must  adopt 
periodic  movements,  so  to  speak,  of  electricity  in  our 
apparatus. 

Faraday's  conception  of  the  electrotonic  state  in  the 
medium  surrounding  wires  carrying  currents  and  mag- 
nets is  thus  made  definite  by  the  quivering  of  the  mag- 
netic lines  of  force.  These  lines  stretch  out  from  the 
poles  of  a  magnet  and  seek  the  shortest  passage  from 
one  pole  to  another.  They  pass  from  the  north  pole  of 
the  earth  to  the  south  pole  through  the  atmosphere,  and 
every  piece  of  iron  or  steel  tends  to  turn  so  that  the 


TRANSFORMATIONS  OF  ENERGY.  129 

greatest  number  of  these  lines  shall  pass  through  its 
substance.  The  oxygen  of  the  air  itself  is  magnetic, 
and  more  lines  of  force  will  pass  through  it  than  pass 
through  copper,  for  instance.  Faraday  believed  that 
all  bodies  are  more  or  less  magnetic,  and  this  has  come 
to  be  the  modern  conception.  The  lines  of  magnetic 
force  pass  through  all  substances  to  a  greater  or  less  ex- 
tent. We  can  conceive  of  a  ship's  compass  turning  so 
as  to  receive  the  greatest  number  of  the  magnetic  lines 
of  the  earth,  and  therefore  pointing  to  the  poles  of  the 
earth.  The  magnetic  lines  of  force  which  stream 
from  one  pole  of  the  earth  to  the  other  are  diverted 
in  a  thousand  directions,  passing  into  the  gas  pipes  and 
water  pipes  of  our  cities,  through  iron-bearing  strata  of 
the  earth,  through  the  'iron  plates  of  the  ocean  steam- 
ships, so  that  it  is  difficult  to  find  even  a  small  space  on 
the  earth's  surface  where  the  lines  of  magnetic  force  are 
parallel  to  each  other  and  of  the  same  intensity.  Instead 
of  making  the  lines  of  magnetic  force  quiver,  we  can 
keep  them  steady  and  produce  electric  currents  by  cut- 
ting them,  so  to  speak,  by  wires.  When  we  revolve  a 
coil  of  wire  near  the  pole  of  a  magnet  we  produce  elec- 
tric currents  in  the  coil.  These  currents  can  be  said  to 
be  produced  by  withdrawing  lines  of  force  from  the  coil 
or  putting  them  into  the  coil.  They  can  also  be  said  to 
be  produced  by  the  turns  of  wire  constituting  the  coil 
cutting  the  lines  of  magnetic  force.  If  we  should  take 
in  both  hands  a  wire  which  is  connected  with  a  galvanom- 
eter and  move  it  rapidly  across  the  pole  of  a  strong  mag- 
net, thus  cutting  the  lines  of  force  which  emanate  from 
the  magnet,  we  would  produce  a  current  of  electricity 
in  this  wire.  Why,  we  do  not  know.  We  could  have 
moved  the  magnetic  pole  instead  of  the  wire,  and  could 
thus  have  produced  the  same  effect.  Electrical  currents 


130  WHAT  IS  ELECTRICITY? 

therefore  result  from  the  relative  movements  of  mag- 
nets and  wires.  We  can  even  light  a  room  by  merely 
revolving  a  large  coil  of  wire  with  great  velocity,  so 
that  it  should  cut  the  magnetic  line  of  the  earth  at  each 
revolution.  The  coil,  however,  would  need  to  be  so 
large  and  the  speed  so  great  that  it  would  be  better  to 
revolve  a  small  coil  between  the  poles  of  a  powerful 
electro -magnet,  as  is  done  in  the  case  of  the  dynamo 
machine.  The  electro-motive  force  of  the  current  ob- 
tained by  cutting  the  magnetic  lines  of  force  is  propor- 
tional to  the  number  of  lines  of  force  that  is  cut  each 
second.  This  electro-motive  force  can  be  thought  of  as 
the  electric  pressure  which  is  obtained  on  the  wire 
through  which  the  electric  current  flows.  The  slightest 
movement  of  a  wire  anywhere  on  the  earth's  surface  can 
be  said  to  produce  an  electro-motive  force  in  the  wire, 
if  such  a  movement  cuts  the  magnetic  lines  of  force  of 
the  earth.  We  therefore  see  the  importance  of  arrang- 
ing the  pole  pieces  of  our  electro-magnets  in  order  that 
we  can  obtain  the  greatest  number  of  lines  of  force  in 
the  space  between  them,  and  so  that  our  revolving  coils 
which  are  to  cut  these  lines  of  force  shall  cut  the  great- 
est number  possible. 

Instead  of  using  strong  permanent  magnets  to  pro- 
duce powerful  magnetic  fields,  we  coil  insulated  copper 
wire  around  bars  of  soft  iron  and  pass  powerful  cur- 
rents of  electricity  through  the  coils,  thus  forming  an 
electro-magnet.  It  is  easy  to  see  how  greatly  we  can 
increase  the  number  of  lines  of  magnetic  force  in  this 
way,  and  it  is  instructive  to  trace  the  building  up  of  a 
strong  magnetic  field  by  means  of  an  electro-magnet. 

If  we  bore  a  number  of  holes  in  a  sensitive  photo- 
graphic plate  and  pass  a  wire  through  them  so  as  to 
form  a  coil,  and  then  place  a  piece  of  iron  in  this  coil,  we 


TRANSFORMATIONS  OP  ENERGY.  13  ± 

have  thus  an  electro-magnet.  On  sprinkling  iron  filings 
on  the  sensitive  plate,  exposing  the  latter  to  the  light,  we 
see  that  the  circular  lines  of  force  around  the  wire 
crowd  through  the  iron  core  of  the  coil,  making  it  a 
powerful  magnet.  The  greatest  number  of  lines  of 
force  we  can  urge  through  a  square  centimetre  of  soft 
iron  is  not  far  from  thirty  thousand.  When  a  large 
number  of  lines  of  force  pass  through  the  air  from  one 
pole  to  another  of  a  magnet  an  appreciable  strain  upon 
the  air  is  noticed.  It  seems  as  if  there  was  a  pull  or 
tension  along  the  lines  of  force  and  a  pressure  at  right 
angles  to  them.  This  pressure  at  right  angles  can 
readily  be  shown  by  suspending  a  glass  rod  between 
wedge-shaped  pole  pieces  of  iron.  The  magnetic  lines 
crowd  across  in  the  air  between  the  wedges  and  the 
glass  rod  moves  aside  to  a  place  of  less  pressure. 

Since  in  dynamos  and  electric  motors  it  is  necessary 
that  the  magnetic  lines  should  pass  from  one  pole  to 
the  other  of  the  electro-magnets  between  the  poles  of 
which  revolve  the  coils  of  the  armature,  and  that  few 
lines  should  be  lost  by  diverging  into  the  air,  we  see  that 
watches  should  not  catch  stray  lines  of  force,  even 
when  quite  near  the  modern  dynamo.  If  one's  watch 
should  be  magnetized  while  one  is  riding  in  an  electric 
car,  it  would  be  a  proof  that  the  electric  motor  propel- 
ling the  car  is  a  very  uneconomical  one.  The  shields 
of  iron  which  are  said  to  protect  certain  watches  from 
being  magnetized  are  generally  inoperative,  for  the  lines 
of  magnetic  force  are  not  entirely  diverted  from  the 
steel  springs  or  detents  of  the  works  of  the  watch.  In 
order  to  be  effective,  such  shields  should  be  of  very  soft 
iron,  nearly  half  an  inch  thick.  No  substance  cuts  off 
the  lines  of  magnetic  force ;  they  pass  through  wood, 
brick  walls,  copper,  and  all  metals.  They  prefer  to  pass 
10 


132  WHAT  IS  ELECTRICITY? 

into  soft  iron  on  their  way  from  one  pole  to  another, 
and  therefore  iron  answers  as  a  species  of  shield.  For 
instance,  a  galvanometer  placed  entirely  within  a  thick 
shell  of  soft  iron  is  used  on  shipboard,  in  laying  sub- 
marine cables,  for  the  purpose  of  testing  the  cable. 
The  little  magnets  of  the  galvanometer  are  unaffected 
by  the  magnetic  condition  of  the  machinery  of  the  ves- 
sel, and  only  respond  to  the  electric  impulses  which  are 
sent  through  the  coils  of  the  galvanometer.  The  reason 
is  that  the  magnetic  lines  of  force  pass  from  the  space 
outside  the  iron  shell  into  the  walls  of  the  shell,  and 
then  out  again  without  crossing  the  air  space  inside  the 
shell,  where  the  galvanometer  is  placed ;  and  none  of 
them  pass  through  the  needle  of  the  galvanometer.  Of 
course  it  would  be  worse  than  useless  to  try  and  protect 
the  ship's  compasses  from  the  magnetism  of  the  ship  by 
inclosing  them  in  iron  shells.  The  lines  of  force  of  the 
earth  would  be  diverted  from  the  needle  around  the 
shell,  and  the  needle  would  no  longer  point  north  and 
south ;  it  would  be  useless  as  a  compass. 

If  we  could  cut  off  the  lines  of  magnetic  force  by 
some  nonmagnetic  substance,  perpetual  motion  would 
be  possible,  for  it  would  be  merely  necessary  to  place  a 
magnet  on  a  wheel  in  the  direction  of  a  diameter ; 
bring  a  powerful  magnet  near  one  pole  of  the  magnet 
on  the  wheel ;  set  the  wheel  in  rapid  revolution,  and 
at  the  instant  the  pole  of  the  magnet  on  the  wheel 
approaches  the  pole  of  the  magnet  which  is  placed  out- 
side the  wheel  insert  automatically  the  supposititious 
shield.  The  magnetic  attraction  being  cut  off,  the 
wheel  carries  the  magnet  around  by  its  inertia  again  to 
the  sphere  of  attraction,  and  the  magnetic  force  is  again 
cut  off,  and  so  on. 

The  steady  lines  of  magnetic  force  can  not  be  cut  off 


TRANSFORMATIONS  OF  ENERGY.        -        133 

by  copper,  but  if  we  make  these  lines  quiver  or  alter- 
nate in  direction  we  shall  find  that  copper  intercepts 
them.  If,  for  instance,  we  place  a  sheet  of  copper  be- 
tween the  pole  of  an  electro-magnet  through  which  a 
powerful  alternating  current  is  passing  and  a  little  coil 
connected  with  an  electric  lamp ;  the  lamp  is  extin- 
guished the  instant  the  copper  sheet  is  interposed  be- 
tween the  coil  and  the  magnet.  In  a  little  while  we  find 
that  the  sheet  of  copper  becomes  very  hot,  and  we  there- 
fore see  that  energy  has  disappeared  in  the  copper.  We 
find  by  experiment  that  little  electric  currents  are  formed 
in  the  mass  of  the  copper  by  the  quivering  of  the  lines 
of  magnetic  force  which  emanate  from  the  electro- 
magnet, and  these  currents  thrust  magnetic  lines  into 
the  field  opposed  to  those  coming  from  the  electro- 
magnet, and  thus  neutralize  their  effect  on  the  little 
coil  of  wire.  We  know,  in  the  case  of  steady  lines  of 
magnetic  force,  that  the  movement  of  a  copper  wire 
across  these  lines  of  force  will  produce  a  current  of 
electricity  hi  the  wire ;  and  that  if  we  keep  the  wire 
still  and  make  or  break  the  current  of  the  electro-mag- 
net, we  shall  also  produce  a  current  in  the  wire.  We 
should  expect,  therefore,  that  a  current  would  flow 
through  the  particles  of  copper  of  a  plate.  When  we 
cause  the  lines  of  magnetic  force  to  quiver  or  alternate, 
every  particle  of  copper  may  be  said  to  cut  the  lines  of 
force.  The  entire  phenomenon  is  one  of  the  excitation 
of  an  electric  current  by  the  movement  of  a  conductor 
across  the  lines  of  magnetic  force  or  the  movement  of 
the  lines  of  force  across  the  conductor.  In  each  case 
a  current  of  electricity  is  excited  in  the  conductor.  If 
we  should  suspend  by  a  fine  fibre  a  light  magnetic  sys- 
tem, made  by  magnetizing  two  cambric  needles,  sticking 
them  into  a  match  so  that  their  opposite  poles  should 


134  WHAT  IS  ELECTRICITY? 

lie  over  each  other,  and  twist  the  strand  by  which  the 
system  is  suspended,  it  will  come  to  rest  sooner  when 
it  revolves  over  a  copper  plate  than  it  does  when  it  re- 
volves far  from  all  neighbouring  objects.  The  poles  of 
the  magnet  nearest  the  copper  plate  induce  currents  of 
electricity  in  this  plate,  and  the  lines  of  magnetic  force 
resulting  from  these  currents  in  the  plate  tend  to  pass 
into  the  revolving  magnet.  The  latter  desires  to 
receive  them,  and  then  tends  to  come  to  rest. 

In  all  these  cases  there  is  a  decay  of  energy  in  the 
copper.  If  we  should  look  upon  the  alternating  electro- 
magnet as  a  source  of  undulations  or  electro-magnetic 
waves  which  it  sends  out  into  the  surrounding  space, 
we  see  that  such  undulations  can  not  pass  through 
what  we  call  good  conductors,  such  as  copper.  If  we 
take  away  the  copper  plate,  which  shields  a  coil  from 
an  electro-magnet  through  which  an  alternating  current 
flows,  and  substitute  a  glass  plate  a  little  lamp  con- 
nected to  the  coil  will  light,  for  there  is  no  decay  of 
energy  in  the  glass ;  it  allows  the  electro-magnetic 
impulses  or  undulations  to  pass  freely  through  it.  With 
reference,  therefore,  to  very  rapid  electro-magnetic  un- 
dulations, we  shall  see  that  our  present  nomenclature  is 
wrong.  To  such  undulations  copper  is  an  insulator 
and  glass  a  good  conductor.  Copper  absorbs  the  energy 
of  the  undulations,  and  glass  does  not. 

Our  attention,  therefore,  has  been  directed  to  the 
distribution  of  lines  of  force  in  and  about  magnets  and 
wires  carrying  currents  of  electricity.  We  have  in 
imagination  filled  space  with  such  lines  which  rep- 
resent the  flow  or  stream,  so  to  speak,  of  what  is 
called  induction.  At  first  sight  it  would  seem  that  we 
have  returned  to  fluid  analogies,  and  that  we  are  about 
to  proclaim  that  all  electro -magnetic  phenomena  are 


TRANSFORMATIONS  OF  ENERGY.  135 

due  to  the  flowing  of  the  ether  into  and  out  of  magnets 
or  around  wires  carrying  currents,  and  that  we  are 
ready  to  assert  that  electricity  is  a  motion  of  the  ether. 
We  shall  see,  however,  that  such  an  assertion  is  certain- 
ly premature,  and  that  at  present  we  can  merely  regard 
the  conception  of  lines  of  force  and  flow  of  magnetic 
induction,  which,  in  other  words,  is  the  number  of  lines 
of  induction  across  any  surface  or  through  any  coil  of 
wire,  as  an  aid  to  our  calculation  of  the  mechanical 
effects  observed. 

The  conception  of  the  flow  of  magnetic  induction  is 
of  great  use  in  our  classification  of  the  vastly  compli- 
cated efforts  which  we  observe  in  the  subject  of  elec- 
tricity. We  say  that  the  north  and  south  poles  of  the 
magnets  attract  each  other,  because  each  pole  tends  to 
embrace  the  greatest  number  of  lines  of  force.  Two 
circles  of  wire  through  which  currents  of  electricity  are 
flowing  in  the  same  direction  turn  their  planes  parallel 
to  each  other,  in  the  same  endeavour  to  embrace  the 
greatest  number  of  lines  of  induction.  They  are  at- 
tracted toward  each  other  in  this  effort.  In  fact,  these 
circles  act  exactly  like  magnets.  If  the  currents  flow  in 
opposite  directions  in  the  two  circles  they  repel  each 
other,  or,  in  other  words,  they  endeavour  to  turn  their 
other  face  around  so  as  to  embrace  the  greatest  number 
of  lines  of  induction  which  are  flowing  in  what  we  call 
a  positive  direction.  If  we  place  soft  iron  in  the  centre 
of  our  little  coils  of  wire  which  are  traversed  by  electric 
currents,  we  magnetize  the  iron — we  create  a  flow  of 
magnetic  induction  through  the  iron.  In  a  permanent 
magnet,  such  as  is  used  in  ships'  compasses,  this  flow  of 
magnetic  induction  is  always  present,  and,  to  fix  our 
ideas,  we  may  perhaps  conceive  of  it  as  a  streaming  of  the 
ether  through  the  iron.  If  we  place  two  north  poles 


136  WHAT  IS  ELECTRICITY! 

of  two  magnets  near  each  other  they  repel  each  other, 
they  tend  to  demagnetize  each  other,  and  the  lines  of 
flow  between  them  is  greatly  weakened.  If  the  mag- 
nets are  free  to  move,  it  is  well  known  that  they  will 
turn  so  as  to  present  a  south  pole  to  a  north  pole ;  or, 
as  we  say  now,  they  will  turn  so  as  to  embrace  between 
them  the  greatest  flow  of  induction.  If  we  endeavour 
to  turn  the  poles  away  from  each  other  we  must  do 
work.  A  force  resists  our  effort  to  separate  a  north 
pole  from  a  south  pole.  We  find  that  this  force  is  pro- 
portional to  the  product  of  the  strength  of  the  two  poles 
divided  by  the  square  of  the  distance  between  them. 
In  the  same  way  we  must  do  work  to  separate  two 
parallel  coils  of  wire  through  which  currents  of  elec- 
tricity are  flowing  in  the  same  direction.  These  parallel 
coils  present  their  north  and  south  pole  to  each  other 
also.  They  are  electro-magnets,  and  the  force  we  must 
overcome  in  separating  them  is  proportional  to  the  prod- 
uct of  the  strength  of  the  currents  which  are  circulating 
in  them.  In  general  we  find  that  magnets  and  coils  of 
wire  in  which  electric  currents  are  circulating  tend  to 
turn  so  as  to  embrace  the  greatest  flow  of  induction 
between  them. 

The  principle  we  have  endeavoured  to  elucidate  in 
the  last  paragraph  is  one  of  the  most  important  in  the 
subject  of  electro-magnetism,  taken  in  connection  with 
the  fact  that  any  change  in  the  flow  of  induction  through 
coils  produces  currents  in  these  coils.  For  instance, 
we  know  that  the  fluctuation  of  a  current  in  one 
coil  will  produce  a  momentary  current  in  a  neigh- 
bouring coil ;  and  the  force  between  the  coils  is  pro- 
portional to  the  product  of  the  two  currents,  and  tends 
to  prevent  the  lessening  of  the  flow  of  induction  be- 
tween the  coils. 


TRANSFORMATIONS  OF  ENERGY. 


137 


A  simple  method  of  showing  that  the  induction  cur- 
rent in  an  induction  coil  on  making  the  circuit  of  the 
primary  is  opposite  in  direction  to  that  in  the  primary, 
and  that  on  breaking  the  circuit  it  is  in  the  same  direc- 
tion, is  as  follows  :  Make  a  file  cut  at  the  centre  of  a 
light  brass  rod  and  balance  the  rod  on  the  edge  of  the 
blade  of  a  pocketknife,  having  previously  hung  two 
scale  pans  made  of 
bristol  board  on  the 
ends  of  the  rod  (Fig. 
13) ;  twist  the  ends  of 
a  piece  of  copper  wire 
together  and  bend  the 
wire  into  the  form  of 
a  ring ;  place  this  ring 
in  one  scale  pan,  hav- 
ing counterbalanced 
it,  and  bring  it  over 
the  end  of  an  electro- 
magnet —  the  latter 
serves  as  the  primary 
coil,  and  the  ring  as  the  secondary  coil ;  on  making  the 
circuit  of  the  primary  the  ring  will  be  repelled  from 
the  electro -magnet,  and  on  breaking  the  circuit  it  will 
be  attracted.  In  the  first  case  two  similar  poles  are 
formed,  and  in  the  second  case  two  opposite  poles.  By 
placing  a  little  mirror  on  the  balance  arm  at  K,  and  by 
the  observation  of  its  movements  by  means  of  a  tele- 
scope and  scale,  a  very  feeble  battery  may  be  employed 
to  excite  the  electro-magnet.  Theoretically,  a  copper 
ring  suspended  above  the  pole  of  the  permanent  magnet 
of  a  receiving  telephone  should  vibrate  when  one  speaks 
into  a  sending  telephone.  Indeed,  telephone  diaphragms 
have  been  made  of  parchment  with  small  closed  circuit 


Fia.  13. 


138  WHAT  IS  ELECTRICITY? 

coils  glued  upon  them  opposite  the  pole  of  the  receiv- 
ing telephone. 

In  1880  the  continuous-current  dynamo  may  be  said 
to  have  reached  its  point  of  perfection.  Its  efficiency 
is  as  high  as  eighty-seven  per  cent,  and  can  be  made 
under  good  conditions  as  high  as  ninety  per  cent — that 
is,  there  was  only  a  loss  of  ten  per  cent  in  the  trans- 
formation from  steam  power  to  the  electrical  current. 
The  dynamo  is  well-nigh  perfect.  The  distribution, 
however,  of  the  current  produced  by  it  is  far  from 
economical  if  such  distribution  is  extended  to  consider- 
able distances.  It  is  found  that  a  light  can  not  be  pro- 
duced at  a  distance  of  ten  miles  from  the  dynamo  with- 
out great  loss  of  energy  along  the  wires  leading  to  the 
lamp.  This  energy  could  be  saved  by  using  massive 
copper  conductors  instead  of  the  ordinary  copper  wires 
which  are  employed  at  short  distances,  but  the  cost  of 
such  great  conductors  is  prohibitive.  In  ordinary  lan- 
guage, it  is  said  that  the  electric  pressure  diminishes 
very  rapidly  with  the  distance;  and  in  the  ordinary 
electric  railway,  such  as  that  of  the  West  End  Eailway 
in  Boston,  Mass.,  it  is  found  necessary  to  feed  the  over- 
head wire  at  various  points  by  auxiliary  conductors  in 
order  to  keep  up  the  electric  pressure,  or  potential.  In 
order,  therefore,  to  substitute  electric  power  for  steam 
on  a  railway  between  Boston  and  New  York,  it  would 
be  necessary  with  a  continuous  current  to  have  power 
stations  every  ten  miles.  For  considerable  distances 
electrical  power  is  hardly  more  economical  than  com- 
pressed air,  and  very  high  authorities  pronounce  in 
favour  of  compressed  air. 

Since  the  dynamo  has  arrived  at  a  high  degree  of  per- 
fection the  attention  of  inventors  is  now  turned  to  the 
important  question  of  the  more  economical  distribution 


TRANSFORMATIONS  OF  ENERGY.  139 

of  the  current  generated  by  the  dynamo,  and  in  the  con- 
sideration of  the  new  methods  of  distribution  we  are  led 
to  a  remarkable  development  of  electrical  science — that 
of  the  varied  uses  of  the  alternating  current.  With  this 
latter  current,  which  flows  to  and  fro,  changing  in  direc- 
tion sixty  to  a  hundred  times  a  second,  like  the  currents 
in  a  telephone — although  the  alternations  produced  by 
the  voice  are  much  faster  than  this  rate — a  hundred 
horse  power  has  been  transmitted  one  hundred  and  ten 
miles  from  Lauffen,  on  the  river  Neckar,  to  Frankfort 
by  means  of  three  wires,  each  about  one  sixth  of  an  inch 
tliick.  The  loss  of  power  in  this  transmission  was  barely 
twenty -five  per  cent,  far  less  than  would  be  the  loss  in 
transmitting  the  same  amount  of  power  five  miles  by  a 
continuous  current.  It  is  proposed  to  use  the  alternat- 
ing system  in  the  transmission  of  power  from  Niagara 
Falls.  It  would  seem  as  if  our  mechanical  contrivances 
to  direct  the  alternating  currents  which  are  produced  by 
every  dynamo  in  the  same  direction  by  means  of  a  com- 
mutator were  working  against  Nature,  and  that  we  are 
returning  to  a  manifestation  which  is  analogous  to  that 
of  the  electro-magnetic  waves  in  the  ether.  If  we 
should  dispense  with  a  commutator,  and  place  instead  of 
it  two  rings  on  a  revolving  shaft  to  which  the  wires  of 
the  coils  which  are  revolving  past  the  poles  of  a  magnet 
are  connected,  we  should  collect  from  these  rings  by  suit- 
able brushes  an  alternating  current.  The  modern  alter- 
nating machine,  therefore,  consists  of  stationary  electro- 
magnets fed  from  a  continuous-current  dynamo,  with  its 
commutator,  and  a  revolving  armature  the  coils  of  which 
are  connected  to  two  rings  on  the  revolving  shaft.  We 
need  the  commutator  on  the  dynamo  which  is  furnish- 
ing the  current  for  magnetizing  the  field  magnets  of 
the  alternating  dynamo.  We  do  not  need,  however,  a 


140  WHAT  IS  ELECTRICITY! 

commutator  to  take  off  the  alternating  currents  produced 
by  the  revolution  of  the  armature  coils  of  the  alternat- 
ing dynamo.  Now,  the  principal  reason  why  the  alter- 
nating current  is  superior  to  the  direct  current  in  the 
transmission  of  power  to  a  distance  resides  in  this :  that 
with  the  alternating  current  we  can  transform  a  com- 
paratively low  electric  pressure  or  voltage  to  an  enor- 
mously high  one,  and  so  overcome  the  electrical  resist- 
ance of  a  long  line  of  comparatively  fine  wire. 

This  transformation  is  accomplished  by  means  of  an 
apparatus  which  is  substantially  the  Ruhmkorff  coil. 
This  consists  of  two  parts  an  electro -magnet  wound 
with  a  few  turns  of  coarse  wire  with  a  core  of  bundles 
of  fine  wires,  and  a  fine-wire  coil  entirely  separate  from 
the  coil  of  the  electro-magnet,  but  wound  closely  upon 
it.  When  a  battery  current  is  made  and  broken  through 
the  electro-magnet  coil,  which  is  termed  the  primary,  a 
flow  or  flux  of  magnetic  induction  is  sent  through  the 
fine-wire  coil ;  a  strong  electro-motive  force  or  pressure 
is  set  up  in  this  coil,  since  we  know  that  the  flux  through 
a  coil  always  produces  a  difference  of  electric  potential 
in  the  coil.  With  a  potential  of  only  four  units  or  volts 
in  the  primary,  we  can  obtain  at  least  ten  thousand 
volts  in  the  secondary  circuit. 


CHAPTER  XL 

ALTERNATING   CURRENTS. 

WE  can  prove  by  the  doctrine  of  the  conservation 
of  energy  that  the  direction  of  the  induced  current  in  a 
coil  neighbouring  to  that  carrying  the  inducing  current 
will  be  opposed  in  direction  to  this  latter  current.  For, 
suppose  that  the  current  in  the  coil  on  the  circuit  A  B 
(Fig.  14)  induces  a  current  in  the  coil  on  the  circuit  C  D 
in  the  same  direction  as  itself,  as  shown  by  the  arrow ; 
on  connecting  the  circuit  A  B  with  the  circuit  C  D,  as 
indicated  by  the  dotted  line, 
we  should  be  able  to  increase 
the  current  flowing  through 
both  coils  by  placing  them 
parallel  to  each  other,  for  we 
should  have  the  induced  cur-  p^ 

rent  superimposed  on  the  in- 
ducing current,  and  in  the  same  direction.  We  should 
get  an  increase  of  energy  merely  by  placing  coils  parallel 
to  each  other  without  doing  any  work.  That  is,  we 
could  get  more  out  of  a  battery  which  is  supplying  the 
current  by  placing  the  coils  in  the  circuit  near  each 
other,  than  we  could  by  placing  them  apart. 

The  doctrine  of  the  conservation  of  energy  can  also 
be  applied  to  the  determination  of  the  direction  of  the 
currents  which  are  produced  by  moving  a  coil  in  a 


142  WHAT  IS  ELECTRICITY! 

magnetic  field.  Let  us  take  the  case  of  the  movement 
of  a  coil  across  the  face  of  the  pole  of  a  magnet. 
When*  we  draw  the  coil  away  from  the  pole  a  current 
is  induced  in  the  coil.  This  current  is  due  to  the 
change  of  the  flow  of  induction  through  the  coil ;  or, 
as  we  ordinarily  say,  it  is  due  to  the  removal  of  lines  of 
force  from  the  coil.  How  the  induced  current  must 
flow  in  such  a  direction  that  the  attraction  of  the  pole 
formed  by  it — the  coil  becoming  for  a  moment  an 
electro-magnet — on  the  pole  of  the  magnet  must  tend 
to  resist  the  movement  of  the  coil ;  for  if  the  pole  of 
the  electro-magnet  repelled  the  pole  of  the  magnet,  we 
should  be  assisted  in  moving  the  coil  away  from  the 
magnet,  and  this  would  be  contrary  to  the  conservation 
of  energy,  for  we  should  be  gaining  more  work  than 
we  are  doing.  Let  us  now  see  what  would  happen  if, 
instead  of  moving  a  coil  from  a  magnet,  we  should 
move  the  magnet  away  from  the  coil.  In  this  case, 
also,  we  see  that  the  current  induced  in  the  coil  must 
be  in  such  a  direction  as  to  resist  the  movement  of  the 
magnet,  and  if  the  coil  were  properly  suspended  it 
would  follow  the  magnet.  A  simple  way  of  trying  the 
experiment  is  as  follows :  Suspend  a  copper  disk  by  a 
single  thread  over  a  magnet  which  can  be  made  to  re- 
volve close  to  it  and  under  the  disk.  As  the  poles  of 
the  magnet  sweep  under  the  disk  currents  of  elec- 
tricity are  induced  in  the  disk,  just  as  if  the  disk  was 
made  up  of  coils  of  wire.  These  currents  flow  in 
such  a  direction  as  to  oppose  the  movement  of  the 
magnet.  They  form  little  electro-magnets,  the  poles 
of  which  attract  the  poles  of  the  moving  magnet,  and 
consequently  the  disk  is  swept  round  with  the  magnet. 
In  this  simple  apparatus  we  have  made  our  first 
acquaintance  with  the  rotary  magnetic  field,  which  is 


ALTERNATING  CURRENTS. 


143 


becoming  of  such  importance  in  the  problem  of  trans- 
mitting power  over  great  distances.  The  copper  disk 
is  the  armature  of  our  motor,  but  we  shall  see  later  that 
it  is  not  necessary  to  use  a  rotating  magnet.  We  can 
produce  the  same  effect  by  alternating  an  electric  cur- 
fent  suitably  through  electro-magnets  which  are  fixed 
beneath  the  disk. 

In  order  to  understand  the  action  of  fluctuating  or 
alternating  currents  upon  each  other,  we  must  consider 
what  is  termed  the  phase  of  the  currents  with  respect 
to  each  other.  It  is  difficult  to  obtain  a  clear  idea 
of  this  word  phase,  but  it  is  necessary  that  we  should 
do  so. 

If,  in  Fig.  15,  a  b  c  d  e  represents  a  wave,  c  being 
the  crest  of  the  wave  and  d  being  the  trough,  and  also 


FIG.  15. 


if  c  and  d  represent  two  boats,  c  will  be  at  the  height 
of  its  upward  movement  when  d  is  at  the  lowest  point. 
The  difference  of  phase  of  the  boats  in  regard  to  their 
relative  moment  is  said  to  be  180°,  for  if  we  draw  a  circle, 
A  B  C  M  (Fig.  15),  we  can  represent  the  motion  of  the 
boats  by  the  relative  movement  of  two  points  around 
the  circle.  The  rise  of  A  to  C  and  the  fall  of  M  to  the 
point  on  the  circle  opposite  C  in  the  same  time  can  be 


144  WHAT  IS  ELECTRICITY  ? 

represented  by  the  curve  a  5  c  d  e,  in  which  the  distances 
along  M  Y  represent  the  times  when  A  and  M  have 
reached  different  points  in  their 
run  around  the  circle.  As  an- 
other illustration,  suppose  we 
have  two  pendulums,  one  of 
which,  C,  Fig.  16,  is  passing 
through  the  middle  point  of  its 
swing,  while  the  other,  D,  is  re- 
turning, having  reached  the  ex- 
treme amplitude  of  its  swing. 
16  These  pendulums  have  a  differ- 

ence of  phase  of  90°,  and  their 

motions  can  be  represented  also  by  the  movements  of 
two  bodies  around  a  circle.  If  the  boats  c  and  d  were 
together,  they  would  rise  and  fall  together  and  would 
be  in  the  same  phase. 

As  a  practical  illustration  of  difference  of  phase,  let 
us  examine  the  working  of  the  telephone.  Is  there 
any  difference  of  phase  between  the  motion  of  the  dia- 
phragm against  which  we  speak  and  that  of  the  dia- 
phragm to  which  we  listen  ?  The  human  voice  sets  the 
iron  diaphragm  in  motion,  and  when  it  is  moving 
swiftest  it  is  causing  the  greatest  disturbance  of  the  lines 
of  induction  between  it  and  the  magnet  of  the  tele- 
phone. The  diaphragm  of  the  receiving  telephone 
starts  into  movement  at  the  instant  of  this  swiftest 
movement  of  the  sending  diaphragm.  It  is  therefore 
in  the  condition  of  a  boat  at  M  (Fig.  15)  while  the 
sending  diaphragm  is  in  the  condition  of  the  boat  at  c. 
There  can  be  a  difference  of  phase  of  90°  between 
them. 

Let  us  now  examine  what  takes  place  when  we  put  a 
copper  ring  in  front  of  an  electro-magnet  through  which 


ALTERNATING  CURRENTS. 


145 


an  alternating  current  is  flowing.  This  alternating 
or  periodic  current  produces  a  fluctuation  in  the  flow 
of  induction  through  the  neighbouring  copper  ring,  and 
an  alternating  current  flows  around  the  ring.  If  there 
were  no  lagging  of  the  current  in  the  ring,  due  to  self- 
induction  or  the  setting  up  of  the  lines  of  force  in  the 
surrounding  medium,  the  conditions  of  movement  would 
be  represented  in  Fig.  17,  in  which  the  strong  line 
represents  the  primary  or  inducing  current  in  the  elec- 
tro-magnet, while  the  thin  line  represents  the  secondary 
or  induced  current  in  the  copper  ring.  The  difference 
of  phase  between  C  and  D  is  90°.  Now  if  the  height 
of  the  two  lines  above  or  below  the  horizontal  line  C  M 
represents  the  strength  of  the  currents  at  any  time,  we 
know  that  the  force  between  the  electro -magnet  and 
the  coil  is  proportional  to  the  product  of  these  currents. 
If,  therefore,  we  should  multiply  together  the  respective 
distances  of  two  points,  such  as  C  D  or  E  F,  above  and 


below  the  middle  line  C  M,  we  should  obtain  the  force 
of  attraction  or  repulsion.  The  strength  of  the  current 
of  induction  at  C  is  nothing,  and  that  of  the  inducing 
current  is  C  D.  Hence,  nothing  (or  zero)  multiplied  by 
C  D  is  nothing,  and  the  dotted  curve  which  represents 
the  resultant  attraction  or  repulsion  starts  from  C,  and 
rises  and  falls  in  the  manner  indicated  by  the  dotted 


146  WHAT  IS  ELECTRICITY? 

line.  We  should  have  alternate  equal  attractions  and 
repulsions.  "Currents  would  be  induced  in  opposite 
directions  to  that  of  the  primary  current  when  the  latter 
current  was  changing  from  a  zero  to  maximum  positive 
or  negative  current,  so  producing  repulsion ;  and  would 
be  induced  in  the  same  direction  when  changing  from 
maximum  positive  or  negative  to  zero,  so  producing  at- 
tractions."* The  ring,  therefore,  would  not  be  at- 
tracted or  repelled  by  the  electro-magnet.  The  current, 
however,  in  the  copper  ring  is  not  in  phase  with  the 
current  in  the  electro-magnet  or  the  inducing  current. 


FIG.  18. 

The  reason  that  it  is  not  in  phase  or  in  step  with  the 
inducing  current  is  due  to  the  work  that  it  must  do  in 
establishing  lines  of  force  about  itself — in  directing,  so 
to  speak,  a>  flow  of  induction  around  its  circuit.  The 
time  that  is  spent  in  this  work  is  different  from  that 
spent  in  the  inducing  circuit  in  similar  work.  The 
crests  and  troughs  of  the  waves  in  each  circuit,  there- 
fore, can  not  be  represented  by  the  thick  and  thin  lines 
of  Fig.  17,  but  are  represented  by  the  lines  of  Fig.  18. 
The  dotted  line  is  obtained,  as  before,  by  multiplying 

*  Prof.    Elihu   Thomson,    Novel    Phenomena   of   Alternating 
Currents. 


ALTERNATING  CURRENTS.  14-7 

together  the  strengths  of  tlie  current  represented  by 
the  distances  of  any  two  points,  such  as  A  and  B — one 
on  the  thick  curve  and  one  on  the  thin  curve — from 
the  central  line  C  M.  "  During  the  period  of  repul- 
sion both  the  induced  and  inducing  currents  have  their 
greatest  values,  while  during  the  period  of  attraction 
the  currents  are  of  small  amounts  comparatively. 
There  is  then  a  repulsion  due  to  the  summative  effects 
of  strong  opposite  currents  for  a  lengthened  period 
against  an  attraction  due  to  the  summative  effects  of 
weak  currents  of  the  same  direction  during  a  shortened 
period,  the  resultant  effect  being  a  greatly  preponder- 
ating repulsion."  A  copper  ring  suspended  opposite 
the  pole  of  an  electro-magnet  through  which  circu- 
late alternating  currents  is  repelled.  I  have  been  ac- 
customed in  my  lectures  to  show  the  following  modi- 
fication of  this  experiment :  A  thin  copper  cylinder  is 
slipped  over  a  number  of  pieces  of  soft  iron  wire  which 
protrude  from  the  centre  of  an  electro-magnet.  "When 
an  alternating  current  is  suddenly  sent  through  the 
electro-magnet,  the  iron  wires  fly  into  the  centre  of  the 
coil,  while  the  copper  cylinder  is  shot  in  the  opposite 
direction  many  feet. 

We  owe  to  Prof.  Elihu  Thomson  a  number  of  similar 
interesting  experiments.  If,  for  instance,  a  copper  ring 
is  held  over  an  electro-magnet  through  which  is  cir- 
culating an  alternating  current,  it  is  repelled,  and  can  be 
made  to  float  in  the  air  without  visible  means  of  support. 
A  thin  copper  sphere  suspended  over  the  pole  of  the 
electro-magnet,  one  half  of  which  is  shielded  by  a  plate 
of  copper,  immediately  begins  to  spin.  Its  rotary  ac- 
tion is  due  to  the  difference  of  phase  between  the  cur- 
rents induced  in  the  copper  plate  and  in  the  sphere. 
Prof.  Thomson  observed  that  if  a  copper  ring  was  held 
11 


148  WHAT  IS  ELECTRICITY? 

in  an  oblique  position  in  front  of  the  pole  of  an  elec- 
tro-magnet, it  tended  to  turn  so  as  not  to  inclose  any 
flux  of  magnetic  induction.  He  therefore  took  a  con- 
tinuous-current armature,  such  as  is  employed  in  the 
continuous-current  dynamo,  and  connected  the  brushes 
by  a  wire.  When  an  alternating  current  passed  through 
the  field  or  stationary  magnets  of  the  dynamo,  the  coils 
of  the  armature  acted  like  the  copper  ring  and  tended 
to  place  themselves  so  as  not  to  embrace  the  flow  of 
magnetic  induction.  This  arrangement  constituted  an 
alternating-current  motor. 

It  is  interesting  to  notice  that  Faraday  observed  this 
phenomenon  of  the  turning  of  the  copper  ring,  and 
Maxwell,  hi  his  Electricity  and  Magnetism,  remarks: 
"  Hence  the  effect  of  magnetic  force  on  a  perfectly  con- 
ducting channel  tends  to  turn  it  with  its  axis  at  right 
angles  to  the  lines  of  magnetic  force — that  is,  so 
that  the  plane  of  the  channel  becomes  parallel  to  the 
lines  of  force.  An  effect  of  a  similar  kind  may  be  ob- 
served by  placing  a  penny  or  a  copper  ring  between  the 
poles  of  an  electro-magnet.  At  the  instant  that  the 
magnet  is  excited  the  ring  turns  its  plane  toward  the 
axial  direction,  but  this  force  vanishes  as  soon  as  the 
currents  are  deadened  by  the  resistance  of  the  copper."  * 

These  interesting  experiments  of  Elihu  Thomson  I 
found,  to  my  surprise,  could  be  performed  with  very 
simple  apparatus.  Although  they  were  discovered  by  the 
use  of  powerful  alternating  currents,  they  can  be  repeated 
with  two  or  three  cells  of  a  battery,  the  current  of  which 
is  made  and  broken  through  an  electro-magnet  by  means 
of  an  ordinary  interrupter,  such  as  is  commonly  used  on 
a  Kuhmkorff  coil  and  known  as  the  hammer-and-anvil 

*  Maxwell's  Electricity  and  Magnetism,  vol.  ii,  §  843. 


ALTERNATING  CURRENTS. 


149 


interrupter.  The  wonder  is  that  these  remarkable 
phenomena  should  have  escaped  observation  so  long.  I 
arrange  the  apparatus  as  follows:  The  coarse  coil 
through  which  the  battery  current  is  rapidly  made  and 
broken  by  the  interrupter  is  first  placed  in  a  horizontal 
position,  having  been  provided  with  a  bundle  of  fine 
iron  wires  for  a  core.  Another  smaller  bundle  of  fine 
iron  wires  (B,  Fig.  19)  is  held  by  a  suitable  clamp 
directly  opposite  the  end  of  the  electro-magnet.  On 


FIG.  19. 

this  latter  bundle  is  slipped  a  bare  copper  wire,  of  which 
the  ends  are  twisted  so  as  to  form  a  light  ring.  This 
ring  is  suspended  by  a  loop  of  silk  thread  from  a  sup- 
port, S.  When  the  current  is  started  in  the  electro- 
magnet the  little  ring  is  instantly  repelled  along  the  iron 
core  B,  and  endeavours  to  set  its  plane  parallel  with  the 
bar  B.  A  heavier  copper  ring  is  more  suitable  than  the 
very  light  ring  to  show  the  turning  action.  The  electro- 
magnet is  then  placed  in  a  vertical  position  (Fig.  20),  a 
thin  copper  disk  is  suspended  over  one  half  of  the  pole 


150 


WHAT  IS  ELECTRICITY? 


of  the  magnet,  and  a  copper  plate  is  placed  partly  be- 
neath the  disk.  When  the  magnet  is  excited  the  disk  is 
set  in  rapid  rotation. 

An  interesting  modification  of 
the  latter  experiment  is,  to  substi- 
tute a  hollow  light  brass  ball,  such  as 
are  used  as  ornaments  on  certain  cur- 
tain fixtures.  The  ball  rapidly  spins 
about  its  axis  of  suspension. 

The  copper  ring  in  this  experi- 
ment gets  very  hot  if  strong  alter- 
nating currents  are  used.  If  it  is 
suitably  placed  in  a  vessel  of  water, 
the  water  can  be  made  to  boil.  We 
have  here  a  transformation  of  elec- 
tric energy  into  motion  and  also  into 
heat.  The  heating  effect  can  also 
be  shown  by  the  employment  of  very  simple  means 
within  the  reach  of  almost  any  experimenter.  To  the 


FIG.  20. 


FIG.  21. 

copper  wire  which  we  have  used  to  show  the  effects  of 
repulsion,  solder  an  iron  wire  at  one  end  of  a  diameter 


ALTERNATING  CURRENTS.  151 

and  a  copper  wire  at  the  other  end  of  this  diameter ; 
connect  these  wires  to  a  suitable  galvanometer  (Fig.  21), 
slip  the  copper  ring  on  the  pole  of  the  magnet,  insulat- 
ing it  from  it  by  a  roll  of  paper ;  when  the  circuit  is 
made  and  broken  through  the  electro-magnet  the  gal- 
vanometer speedily  shows  that  the  copper  ring  is  heat- 
ing. We  have  in  this  case  a  thermal  juncture  of  iron 
and  copper  on  the  ring  and  the  other  juncture  outside 
the  ring.  Two  cells  of  a  battery  will  readily  show  this 
phenomenon.  The  ring  is  a  step-down  transformer  of 
very  small  resistance,  combined  with  a  very  small 
electro-motive  force,  and  with  consequently  a  compara- 
tively large  current. 


CHAPTER  XII. 

TRANSMISSION   OF   POWER   BY   ELECTRICITY. 

WE  have  pointed  out  that  the  force  of  gravitation 
affords  us  our  practical  measures  of  electricity.  It  can 
also  produce  electricity  by  means  of  the  weight  of  water. 

In  1877  Sir  William  Siemens,  in  his  presidential  ad- 
dress before  the  Iron  and  Steel  Institute  of  Great  Brit- 
ain, spoke  of  the  possibility  of  utilizing  the  power 
wasted  in  the  falls  of  Niagara,  and  said  :  "  Time  will 
probably  reveal  to  us  effectual  means  of  carrying  power 
to  great  distances,  but  I  can  not  refrain  from  alluding 
to  one  which  is,  in  my  opinion,  worthy  of  considera- 
tion— namely,  the  electrical  conductor.  A  copper  rod 
three  inches  in  diameter  would  be  capable  of  trans- 
mitting 1,000  horse  power  to  a  distance,  say,  of  30  miles." 
Again,  in  1878,  he  states  that  there  would  be  sixty  per 
cent  lost  in  transmitting  this  amount  of  power  by  elec- 
trical means  over  a  distance  of  30  miles. 

In  the  year  1882  M.  Depretz  attempted  to  transmit 
power  from  Weissbach  to  Munich  over  35  miles  of  iron 
telegraph  wire  0'18  inch  in  diameter.  He  used  a 
dynamo  such  as  is  commonly  employed  to-day  on  arc- 
light  circuits,  and  the  pressure  which  forced  the  elec- 
tricity, in  common  parlance,  along  the  wires  was  1,500 
units,  or  volts,  as  they  are  termed.  It  is  important  to 
notice  the  amount  of  this  pressure,  for  in  modifying  this 


TRANSMISSION  OP  POWER  BY  ELECTRICITY.  153 

factor  greater  success  has  been  reached.  The  increase 
in  the  pressure  seems  to  be  the  key  to  the  entire  situa- 
tion. The  first  experiment  of  Depretz  was  not  entirely 
satisfactory,  and  it  was  repeated  in  1883 ;  but  the  second 
experiment  was  far  from  being  successful.  In  1883  still 
another  experiment  was  made,  which  was  far  in  advance 
of  previous  experiments.  Power  was  transmitted  from 
Yizille  to  Grenoble,  in  France,  a  distance  of  8|  miles, 
with  a  silicium  bronze  wire  of  0-079  inch  in  diameter. 
Seven  horse  power  was  obtained  at  the  receiving  end, 
the  loss  being  only  sixty-two  per  cent.  The  improve- 
ment resulted  from  employing  3,000  volts  instead  of 
1,500.  Here  it  was  clearly  indicated  that  the  direction 
in  which  to  work  was  in  employing  high  electrical  pres- 
sure or  voltage. 

It  was  soon  discovered  that  advance  was  barred  in 
this  direction  of  increasing  the  pressure  by  the  impossi- 
bility of  making  a  dynamo  which  would  furnish  the 
high  pressure  with  a  continuous  current,  such  as  is  com- 
monly employed  to-day  on  our  street  arc  lights.  In  a 
subsequent  experiment  M.  Depretz  endeavoured  to  con- 
struct a  dynamo  which  would  furnish  a  higher  voltage ; 
and  although  the  experimental  dynamo  could  not  furnish 
the  high  voltage  of  6,000  units  which  was  desired  and 
was  burned  out  in  the  experiments,  nevertheless  M.  De- 
pretz showed  that  52  horse  power  could  be  transmitted 
35  miles  over  a  copper  wire  0'2  inch  thick.  Although 
this  latter  experiment  was  a  failure,  it  clearly  showed 
how  greater  success  could  be  obtained.  A  higher  pres- 
sure or  voltage  must  be  used,  and  instead  of  a  direct- 
current  dynamo  a  new  type  must  be  employed — namely, 
an  alternating-current  dynamo,  one  in  which  the  elec- 
trical current  pulsates  to  and  fro,  now  in  one  direction 
and  now  in  the  opposite. 


154:  WHAT  IS  ELECTRICITY! 

It  will  be  noticed  that  during  the  years  1877-'83 
men's  minds  had  changed  greatly  in  considering  the  sub- 
ject of  the  transmission  of  power  from  Niagara  Falls  to 
even  the  distance  of  30  miles.  The  early  objectors  to 
the  scheme  calculated  the  expense  of  the  enormous 
copper  conductor  three  inches  in  diameter,  and  showed 
the  practical  impossibility  of  the  plan.  The  later  ob- 
jectors pointed  out  that,  although  power  from  the  falls 
could  be  transmitted  30  miles  over  a  wire  only  0'2  of 
an  inch  in  diameter  instead  of  three  inches  in  diameter, 
no  dynamo  could  be  constructed  which  could  give  the 
large  pressure  of  6,000  units  and  maintain  its  life.  It 
would  be  burned  out  by  the  excess  of  its  emotion. 

The  remarkable  new  developments,  therefore,  in  the 
subject  of  the  transmission  of  power  by  electricity  come 
from  the  employment  of  a  high  electrical  pressure  or 
voltage  generated  by  a  to-and-f  ro  or  alternating  current 
instead  of  a  direct  current ;  and,  strange  to  say,  an  ap- 
paratus which  has  long  been  used  on  professors'  tables 
to  illustrate  the  conversion  of  a  low-pressure  current  of 
electricity  into  a  high-pressure  current  has  now  come  to 
have  a  great  commercial  value ;  this  apparatus  is  the 
Ruhmkorff  coil.  In  its  elements  we  have  seen  it  consists 
merely  of  two  coils  of  wire  entirely  separate  from  each 
other,  which  are  slipped  on  a  bundle  of  iron  wire  which 
forms  a  core.  If  an  alternating  or  rapidly  interrupted 
current  of  electricity  from  a  battery  or  a  dynamo  is 
sent  through  one  of  the  coils,  a  current  is  generated 
by  induction  in  the  neighbouring  coil.  The  pressure 
in  this  latter  coil  depends  largely  upon  the  number 
of  windings  in  it.  A  pressure  of  only  two  units  in 
the  coil  which  is  connected  with  the  battery  or  dy- 
namo can  be  exalted  to  a  pressure  of  12,000  to  100,- 
000  units  in  the  independent  coil  by  properly  increas- 


TRANSMISSION  OF  POWER  BY  ELECTRICITY.  155 

ing  its  windings.  Again,  if  the  sparks  from  a  Leyden 
jar  which  is  charged  to  100,000  volts  by  an  electrical 
machine  should  be  sent  through  a  coil  of  many  wind- 
ings, a  neighbouring  independent  coil  can  be  made  to 
give  a  pressure  of  only  four  volts,  with,  however,  a  large 
current  for  an  instant.  The  Leyden-jar  current  is  a 
very  feeble  one  although  it  has  a  high  pressure. 

If  it  were  possible  to  direct  a  bolt  of  lightning 
through  a  coil  of  many  thousand  windings  of  fine 
wire,  a  neighbouring  coil  of  few  turns  of  coarse  wire 
entirely  unconnected  with  the  coil  through  which  the 
bolt  of  lightning  passes  could  be  made  to  furnish  a 
current  for  an  instant  which  would  decompose  water 
into  oxygen  and  hydrogen,  although  this  would  have 
been  out  of  the  power  of  the  original  bolt  of  lightning. 
In  this  system  of  exalting  the  pressure  by  employing 
induction  in  independent  circuits — for  instance,  between 
two  coils  slipped  upon  an  iron  rod  or  bundle  of  wire — 
we  have  the  modern  transformer  system,  which  marks 
a  great  development  in  the  subject  of  the  practical 
application  of  electricity.  It  will  be  noticed  that  we 
have  a  step-up  transformer  when  we  exalt  a  pressure 
of  four  volts  to  100,000  volts,  and  a  step-down  trans- 
former when  we  use  a  pressure  of  100,000  volts  in  a 
fine-wire  coil  to  produce  a  pressure  of  four  volts  in 
a  neighbouring  coil  of  coarse  wire.  A  dynamo,  there- 
fore, producing  only  a  1,000  volt  pressure  and  the 
dynamo  ordinarily  used  to-day  safely  can  stand  this 
pressure,  can  be  employed  to  send  an  alternating  or 
pulsating  current  through  the  coarse  coil  which  we  slip 
upon  our  iron  rod,  and  the  fine-wire  coil  of  greater 
number  of  turns  which  is  placed  near  but  unconnected 
with  the  coarse  coil  can  be  made  to  give  by  induction  a 
pressure  of  from  12,000  to  20,000  volts.  This  pressure 


156  WHAT  IS  ELECTRICITY? 

can  be  transmitted  over  a  fine -wire  one  hundred  miles, 
passed  through  another  fine  wire  coil  upon  an  iron  rod, 
and  a  neighbouring  coarse  coil  can  be  then  made  to 
yield  a  lower  pressure  suitable  for  decomposing  water, 
running  an  electric  motor  without  burning  it  up,  or 
doing  any  work  which  the  generating  dynamo  at  the 
sending  end  is  capable  of. 

In  the  ordinary  use  of  step-down  transformers  for 
electric  lighting  it  is  customary  to  so  arrange  the  ratio 
between  the  number  of  turns  of  wire  in  the  primary  to 
those  in  the  secondary  that  an  electro-motive  force  of  a 
thousand  volts  in  the  primary  is  transformed  to  fifty  or 
seventy-five  volts  in  the  secondary.  A  thousand  volts 
is  therefore  present  in  the  street  circuit,  and  a  current 
of  only  fifty  volts  enters  one  house.  A  thousand- volt 
circuit  would  be  highly  dangerous  in  a  house,  while 
one  of  fifty  volts  is  comparatively  safe.  Such  a  system 
of  transformation  is  far  more  flexible  than  the  system 
of  electric  lighting  by  a  direct  continuous  current,  in 
which,  of  course,  no  transformers  can  be  used,  for  the 
essence  of  the  action  of  a  transformer  lies  in  the  fluc- 
tuating flow  of  induction  produced  by  an  alternating  or 
to-and-fro  current  of  electricity.  With  a  transformer 
one  can  obtain  one  light  or  a  hundred  or  more,  while 
with  a  continuous  current  one  can  not  obtain  one  light 
advantageously  without  having  a  number  lighted  in  the 
same  circuit.  By  properly  proportioning  the  trans- 
formers on  a  one-thousand-volt  circuit  one  can  range 
from  the  production  of  electric  sparks  to  the  produc- 
tion of  an  electric  light  of  sixteen-candle  power  or  to 
the  production  of  the  minute  lamp  that  surgeons  use 
to  light  up  the  human  throat. 

This  great  range  illustrates  the  wonderful  field  of  the 
transformation  of  energy  which  the  study  of  electricity 


TRANSMISSION  OF  POWER  BY  ELECTRICITY.  157 

opens  to  us.  It  seems  as  if  the  nearer  we  get  to  the 
rate  of  pulsation  of  the  electro-magnetic  waves  in  the 
ether  the  nearer  we  get  to  an  economical  employment 
of  this  great  source  of  energy. 

Since  we  can  obtain  an  abundance  of  light  by  a 
transformer,  the  question  naturally  arises,  Can  we  not 
obtain  great  manifestations  of  heat  by  properly  arrang- 
ing the  proportions  between  the  primary  and  secondary 
coils  ?  This  has  been  done  by  Prof.  Elihu  Thomson  in 
a  remarkable  process  called  electric  welding.  Suppose 
that  the  secondary  wire  consists  of  merely  a  copper 
ring  of  very  small  resistance,  and  that  the  electro-motive 
force  excited  in  it  is  one  volt  or  one  unit :  the  ap- 
proximate expression  for  the  strength  of  the  current  in 
the  ring  is  the  electro-motive  force  divided  by  a  very 
small  quantity  made  up  of  the  resistance  of  the  copper 
ring  and  another  factor  depending  on  the  rate  of  alter- 
nation of  this  current  and  what  is  called  the  coeffient  of 
induction.  One  will  therefore  be  divided  by  a  small 
fraction.  We  may,  for  instance,  have  one  volt  divided 
by  xinnr  an<^  thus  obtain  a  current  of  one  thousand  am- 
peres or  units.  This  current  would  speedily  melt  a 
bar  of  iron  of  the  size  of  the  average  human  wrist. 
Prof.  Thomson  by  ingenious  mechanism  has  made  it 
possible  to  weld  different  metals  together,  so  that  the 
strength  at  the  joint  is  superior  to  that  at  other  por- 
tions of  the  rods.  Metals  can  be  welded  together  which 
can  not  be  joined  by  brazing  or  soldering.  The  process 
enables  one  to  apply  great  heat  at  exactly  the  point 
where  we  wish  it.  The  two  bars  to  be  welded  are 
brought  together  end  to  end.  The  principal  resistance 
being  thus  at  the  imperfect  contact  of  these  ends,  great 
heat  is  developed,  the  bars  are  raised  to  a  white  heat  at 
their  junction,  and  they  are  then  pushed  together. 


158  WHAT  IS  ELECTRICITY? 

One  of  the  ingenious  applications  of  this  transforma- 
tion of  energy  is  to  the  welding  together  of  shells  used 
in  warfare  and  to  the  annealing  of  steel  armour  plate 
in  places  where  bolts  or  nuts  are  necessary.  The  steel 
plates  are  so  hard  that  they  resist  the  action  of  ordinary 
tools,  and  they  therefore  have  to  be  softened  in  order 
to  allow  the  boring  for  the  bolts.  The  circuit  of  the 
secondary  of  the  transformer  can  be  closed  at  these 
points  by  resting  the  ends  of  the  transformer  ring  on 
the  iron.  Great  heat  is  therefore  developed  at  such 
points,  and  the  steel  can  be  annealed  at  these  points. 
It  is  evident  that  houses  could  be  heated  by  a  similar 
transformation,  for  coils  of  wire  might  be  so  arranged 
that  currents  of  air  in  passing  over  them  could  readily 
be  heated.  This  transformation  at  present  is  somewhat 
expensive. 

In  1883  the  attention  of  mankind  was  directed  to 
producing  dynamos  which  would  give  the  steadiest  con- 
tinuous current ;  to-day  we  are  striving  to  produce 
dynamos  which  will  give  to-and-fro  currents,  for  the 
success  of  the  new  method  of  transforming  electricity 
from  one  pressure  to  another  depends  upon  the  fluctua- 
tion of  a  current  and  not  upon  its  steadiness.  It  may 
be  said  that  our  inventors  unconsciously  have  been 
imitating  Nature;  for  every  bolt  of  lightning  is  not 
one  continuous  discharge,  but  is  an  alternating  current 
which  pulsates  to  and  fro  ten  or  twelve  times,  or  even 
more,  in  a  millionth  of  a  second. 

It  is  well  known  that  the  ordinary  electric  car  is 
propelled  by  a  dynamo  which  is  similar  to  the  dynamo 
at  the  central  station,  which  generates  the  continuous 
current  of  electricity  utilized  by  the  dynamo  motor  in 
the  car.  The  dynamo  motor  can  be  made  to  generate 
a  continuous  current  of  electricity  if  necessary.  One 


TRANSMISSION  OF  POWER  BY  ELECTRICITY.  159 

dynamo  can  thus  be  said  to  be  the  counterpart  of  the 
other.  The  motor  can  be  made  the  generator  or  the 
generator  the  motor.  This  is  true,  speaking  in  general 
terms.  If  we  use  an  alternating-current  generator,  we 
can  not  use  the  ordinary  motor  such  as  is  now  used  on 
electric  cars.  There  must  be  a  similar  correspondence 
to  that  which  characterizes  generators  and  motors  when 
continuous  currents  are  employed  ;  the  alternating  gen- 
erator requires  an  alternating  motor. 

The  recent  success  in  transmitting  power  over  one 
hundred  miles  is  due  not  only  to  the  method  of  trans- 
forming a  current  of  low  pressure  to  a  high  pressure 
and  then  transforming  back  at  the  receiving  end,  but 
also  to  the  invention  of  an  alternating  motor.  At  first 
sight  it  seems  impossible  that  an  alternating  current 
flowing  through  a  dynamo  could  make  the  armature  of 
the  latter  revolve,  for  the  pole  of  the  electro-magnets 
which  attract  the  armature  become  alternating  north 
and  south  poles  and  apparently  neutralize  each  other's 
effect  on  the  armature.  Here  we  are  brought  again  to 
see  an  apparatus  which  has  long  been  exhibited  on  lec- 
ture tables  developed  into  an  important  commercial 
engine. 

Arago  showed  that  a  copper  disk  suspended  by  a 
thread  above  a  revolving  magnet  would  be  dragged 
around  by  this  magnet.  Its  motion  was  due  to  the  re- 
actions on  the  magnet  of  the  currents  formed  in  the 
copper  by  the  motion  of  the  magnet.  Prof.  Ferraris,  in 
Italy,  and  Mr.  Tesla,  in  America,  showed  that,  instead 
of  rotating  a  magnet,  magnetic  poles  could  be  formed  at 
different  points  in  a  collection  of  electro-magnets  by 
means  of  alternating  currents  in  such  a  manner  as  com- 
pletely to  imitate  a  revolving  magnet. 

The  copper  disk  would  therefore  follow  the  chang- 


160  WHAT  IS  ELECTRICITY! 

ing  poles.  Instead  of  a  disk  an  armature  consisting  of 
copper  rods  was  made  and  the  new  alternating-current 
motor  sprung  into  existence.  Its  most  striking  pecul- 
iarity consists  in  its  absence  of  brushes,  such  as  are 
used  on  ordinary  dynamos.  Its  armature,  or  revolving 
part,  is  practically  a  copper  disk  revolving  under  ex- 
actly the  same  conditions  as  in  Arago's  experiment. 
To  this  or  to  a  similar  alternating-current  motor  we 
must  look  for  aid  in  transmitting  power  great  distances 
over  a  wire. 

It  is  interesting  to  note  that  the  conditions  for  the 
transmission  of  power  by  electrical  means  over  long 
distances  are  closely  analogous  to  those  which  are  em- 
ployed in  long-distance  telephony.  The  transmitter  is 
an  alternating-current  machine  operated  by  the  human 
voice,  and-  the  telephone  at  the  receiving  end  is  an 
alternating- current  motor  which  impresses  its  motion 
upon  the  air  and  thus  reproduces  the  speaker's  voice 
a  thousand  miles  away.  In  long-distance  telephony, 
too,  the  step-up  transformer  is  used. 

By  the  method  of  step-up  and  step-down  trans- 
formers, which  we  have  described,  one  hundred  horse 
power  has  been  transmitted  one  hundred  miles  from 
Lauffen  to  Frankfort  over  a  wire  which  resembles  in 
size  an  ordinary  telegraph  wire.  Although  the  expense 
of  providing  copper  conductors  of  great  size  (for  in- 
stance, three  inches  in  diameter)  has  been  obviated, 
another  difficulty  now  remains — that  of  properly  insu- 
lating the  line  which  is  conveying  a  pressure  of  12,000 
to  20,000  volts.  The  higher  the  pressure  the  more 
tendency  there  is  for  electricity  to  escape  from  the  line. 
In  fact,  a  spark  could  be  obtained  by  presenting  one's 
knuckles  to  the  wire  charged  to  a  pressure  of  20,000 
volts,  and  therefore  the  tendency  of  the  current  to 


TRANSMISSION  OP  POWER  BY  ELECTRICITY.  161 

leave  the  wire  at  each  support  and  escape  to  the  ground 
is  very  great.  The  wire  between  Lauffen  and  Frank- 
fort was  carefully  insulated  at  the  poles,  carrying  it  by 
various  devices,  in  one  of  which  insulating  oil  was 
used.  The  pressure  of  12,000  volts  which  was  actually 
used  between  Lauff e'n  and  Frankfort  was  dangerous  to 
human  life.  A  skull  and  crossbones  was  painted  on 
the  poles  which  carried  the  wires,  and  no  one  cared  to 
touch  the  wires.  It  is  evident  that  if  12,000  to  20,000 
volts  are  to  be  used  on  overhead  wires  in  the  future 
they  must  be  as  much  respected  as  the  way  along  which 
an  express  train  travels. 

It  seems  to  me  undoubtedly  true  that  a  diminution 
in  our  coal  supply  would  result  in  making  the  trans- 
mission of  power  from  Niagara  Falls  to  New  York  a 
success.  Mankind  has  often  cast  uneasy  glances  at 
these  unemployed  giants — waterfalls  and  tides — as  if 
envying  them  their  freedom.  If  we  could  make  them 
store  up  their  energy  in  a  practical  form,  we  should  no 
longer  be  compelled  to  delve  in  mines  a  mile  under  the 
earth  for  coal,  and  we  could  fold  our  hands  while  the 
water  ran  on  and  did  our  work. 

When  the  storage  battery  was  invented  it  was 
thought  that  Niagara  Falls  had  lost  its  freedom  and 
would  be  immediately  set  to  work.  There  was  a  period 
when  the  elation  of  mankind  grew  less  and  faith  in 
storage  batteries  declined  ;  for  they  were  far  from  per- 
fect and  could  not  be  relied  upon.  It  was  much  as  if 
the  swift  trotting  horse  had  appeared  before  long  ex- 
perience had  been  obtained  in  training  him  and  in 
properly  nourishing  him.  The  early  experimenters  en- 
deavoured to  drive  the  storage  batteries  too  hard,  and 
the  battery  broke  down  under  severe  usage.  To-day 
the  storage  batteries  are  becoming  a  commercial  sue- 


162  WHAT  IS  ELECTRICITY? 

cess,  and  people  understand  better  how  to  keep  them  in 
condition.  There  is  therefore  a  possibility  of  employ- 
ing them  to  convey  a  portion  of  the  power  of  Niagara 
to  Chicago.  Let  us  see  what  this  possibility  is. 

Roughly  speaking,  six  horse  power  can  be  stored  in 
a  ton  of  material  which  constitutes  the  storage  battery. 
The  equivalent  of  fifty  horse  power  could  be  certainly 
carried  in  one  freight  car,  and  it  would  therefore  re- 
quire one  hundred  freight  cars  to  transport  5,000  horse 
power  from  Niagara  Falls  to  Chicago.  The  cost  of  the 
batteries  would  be  in  the  neighbourhood  of  $2,000,000, 
and  in  order  to  maintain  5,000  horse  power  in  Chicago 
relays  of  batteries  would  have  to  be  employed.  Against 
the  expense  of  this  method  we  must  place  the  cost  of 
the  high  insulation  of  a  line  of  four  hundred  miles 
under  a  pressure  of  20,000  volts.  In  both  cases,  neg- 
lecting the  factor  of  sublimity,  it  would  be  more  eco- 
nomical to  generate  the  electricity  at  Chicago  from 
coal  by  the  ordinary  method  of  emptying  a  steam  en- 
gine to  drive  a  dynamo.  Improvements  in  construct- 
ing high  insulation  wires  and  in  transforming  high 
electrical  pressures  to  low  ones  and,  vice  versa,  may, 
however,  make  a  greater  revolution  in  the  subject  than 
has  been  made  since  the  first  experiments  of  Depretz, 
in  1882. 

It  is  interesting  to  note  in  connection  with  the  plan 
of  utilizing  the  power  of  Niagara  Falls,  that  some  of 
our  most  thriving  manufacturing  centres  are  not  located 
on  water  courses,  and  depend  upon  coal  and  not  upon 
water  power.  The  cities  of  Fall  Eiver  and  New  Bed- 
ford in  Massachusetts  are  rivalling  Lawrence  and  Lowell 
in  the  number  of  their  spindles.  Both  can  obtain  their 
supply  of  coal  by  cheap  transportation  in  vessels.  The 
cost  of  regulating  water  power  is  a  large  item  in  the 


TRANSMISSION  OF  POWER  BY  ELECTRICITY.  163 

use  of  it,  and  steam  is  found  to  be  more  reliable  and 
cheaper  in  many  instances.  There  is  no  doubt  that  it 
would  be  more  economical  to  generate  5,000  horse 
power  in  Chicago  by  means  of  coal  than  to  attempt  to 
transmit  it  from  Niagara  Falls.  A  great  change,  how- 
ever, in  our  coal  supply  would  speedily  turn  attention 
to  the  immense  waste  of  energy  which  is  going  on  at 
Niagara  Falls,  and  might  convert  New  York  State  into 
a  beehive  of  industries. 


CHAPTER  XIII. 

SELF-INDLTCTION. 

WITH  the  use  of  alternating  or  periodic  currents  of 
electricity  the  phenomenon  of  self-induction  becomes 
of  great  importance,  and  it  is  desirable  to  obtain  a  clear 
conception  of  it.  The  quantity  we  call  self-induction 
of  a  wire  or  coil  is  generally  small  compared  with  its 
resistance.  The  larger  the  self-induction,  the  more 
slowly  will  a  continuous  electric  current  rise  to  its 
stationary  value. 

The  self-induction  is  measured  by  the  number  of 
lines  of  force  established  by  the  current.  To  send  out 
these  lines  of  force  requires  an  expenditure  of  energy 
which  is  measured  by  the  product  of  the  lines  of  force 
due  to  a  unit  current  multiplied  by  the  square  of  the 
current,  and  this  multiplied  by  one  half.  This  energy 
produces  a  stress  in  the  medium  surrounding  the  wire 
or  coil.  The  greater  the  self-induction,  the  longer  the 
time  it  takes  to  bring  a  current  up  to  a  certain  value. 
When  this  energy  which  is  stored  in  the  medium  is 
allowed  to  return  to  a  wire  which  is  coiled  around  a 
bar  of  iron,  the  magnet  thus  formed  can  be  demagnet- 
ized. This  can  be  accomplished  by  suddenly  placing  a 
wire  of  small  resistance  across  the  poles  of  the  battery 
which  is  exciting  the  electro-magnet.  The  lines  of 
force  are  thus  withdrawn  from  the  field. 


SELF-INDUCTION.  165 

If  we  have  a  large  amount  of  self-induction  in  the 
field  of  an  electro-magnet — in  other  words,  if  we  must 
generate  a  large  number  of  lines  of  force  to  bring  the 
medium  around  the  magnet  up  to  a  high  degree  of 
stress — we  must  do  a  large  amount  of  work.  In  the 
case  of  the  electro-magnets  now  daily  employed  in 
commercial  operations  this  energy  is  measured  in  tons 
lifted  through  several  feet. 

An  idea  can  be  obtained  of  the  energy  manifested 
in  the  phenomenon  of  self-induction  by  observing  the 
great  spark  and  flash  of  light  which  manifests  itself 
when  the  trolley  leaves  the  overhead  wire  or  when  the 
trolley  wire  breaks.  This  is  caused  by  the  magnetic 
energy  which  was  stored  up  in  the  medium  around  the 
wire  and  motor  of  the  car  rushing  back  from  the 
medium  into  the  wire  and  decaying  there  in  the  form 
of  heat.  The  heat  developed  is  sufficient  to  melt  the 
surface  of  the  copper  wire  constituting  the  trolley  wire. 
It  would  take  several  horse  power  applied  directly  for 
the  same  instant  of  time  to  produce  the  same  heating 
effect. " 

When  we  consider  the  phenomenon  of  self-induc- 
tion we  perceive  that  the  early  attempts  to  obtain  the 
velocity  of  electricity  were  nugatory,  for  different 
values  of  what  appeared  to  be  the  velocity  of  propa- 
gation could  be  obtained  with  different  lengths  of  the 
electric  circuit.  On  account  of  the  time  that  is  neces- 
sary to  establish  the  strain  in  the  medium  along  a  tele- 
graph wire  containing  the  electro-magnets  used  in 
sending  signals,  the  maximum  value  of  the  current 
which  actuates  the  recording  apparatus  does  not  arrive 
until  a  certain  period  has  elapsed  after  the  sending-key 
is  touched.  This  period  of  time  depends  upon  the 
self-induction  or  inductance  of  the  circuit;  different 


166  WHAT  IS  ELECTRICITY! 

values  of  what  seems  to  be  the  velocity  of  electricity 
will  therefore  be  obtained  with  every  circuit. 

The  early  workers  in  the  subject  of  electricity 
confined  their  attention  to  what  they  called  electricity 
as  it  manifested  itself  on  conductors.  The  idea  of  a 
strain  in  the  medium  surrounding  the  conductors,  or  of 
energy  being  stored  up  in  this  medium,  did  not  take 
firm  hold  of  men's  minds  until  Maxwell  stated  his 
great  hypothesis  of  the  electro-magnetic  origin  of  light. 
Meanwhile  the  transformations  of  energy  manifested  in 
the  ever-increasing  phenomena  of  electricity  demanded 
the  hypothesis  of  a  medium. 

The  electric  energy  which  propels  an  electric  car 
resides  in  the  medium  around  the  trolley  wire,  and  the 
rate  of  decay  of  this  energy  along  the  wire  is  what  we 
call  the  electric  current. 

The  energy  of  the  dynamo,  thus,  is  not  trans- 
mitted along  the  wire  ;  it  is  manifested  hi  the  ether  of 
space  and  in  the  insulating  media  about  the  wire. 
Lodge,  in  his  Modern  Yiews  of  Electricity,  remarks : 
"  The  energy  of  a  dynamo  does  not,  therefore,  travel  to 
a  distant  motor  through  the  wires,  but  through  the  air. 
The  energy  of  an  Atlantic  cable  battery  does  not  travel 
to  America  through  the  wire  strands,  but  through  the 
insulating  sheath.  This  is  a  singular  and  apparently 
paradoxical  view,  yet  it  is  well  founded." 

On  Poynting's  hypothesis,  an  electrical  current 
should  start  first  at  the  boundary  of  the  wire  near  the 
insulating  substance.  To-and-fro  currents,  like  those 
from  a  discharge  of  lightning,  should  be  confined  to  the 
outside  of  the  conductor.  This  is  found  to  be  true. 
"With  such  currents,  therefore,  a  hollow  conductor  is 
better  than  a  solid  one.  With  an  extremely  rapid  rate 
of  oscillation,  far  into  the  billions  per  second,  the  elec- 


SELF-INDUCTION.  Igf 

trical  current  leaves  the  wire,  and  may  be  said  to  travel 
through  the  medium.  It  then  manifests  itself  as  light, 
and  the  copper  is  opaque  to  light.  In  speaking  of  this 
skin  action,  Lodge  remarks  that  this  action  is  greatly 
broken  up  by  making  the  conductor  of  bundles  of  wire, 
in  order  to  afford  the  medium  surrounding  the  wire 
access  to  every  part  of  the  conductor.  It  is  well  also 
to  make  the  conductor  expose  a  large  surface  to  the 
dielectric. 

"  A  lightning  conductor,  therefore,  should  not  be  a 
round  rod,  but  a  flat  strip,  or  a  strand  of  wires,  with  the 
strands  as  well  separated  as  convenient.  I  might  go 
on  to  say  here  that  iron  makes  an  enormously  worse 
conductor  than  copper  for  rapidly  alternating  currents ; 
so  it  does  for  currents  which  alternate  with  moderate 
rapidity — a  few  hundred  or  thousand  a  second — like 
those  from  an  alternating  dynamo  or  a  telephone ;  but, 
singularly  enough,  when  the  rapidity  of  oscillation  is 
immensely  high,  as  it  is  in  Leyden-jar  discharges  and 
lightning,  iron  is  every  bit  as  good  as  copper,  because 
the  current  keeps  to  the  extreme  outer  layer  of  the  con- 
ductor, and  so  practically  does  not  find  out  what  it  is 
made  of."  * 

I  have  examined  the  phenomenon  of  electrical  oscil- 
lations on  iron  wire,  and  I  find  that  Prof.  Lodge's  con- 
clusions are  subject  to  certain  limitations.  Oscillations 
as  rapid  as  ten  million  a  second  can  magnetize  iron ; 
and  the  period  of  the  waves  on  iron  is  also  affected  by 
the  magnetic  nature  of  the  iron. 

I  have  employed  in  my  lectures  the  following 
method  of  illustrating  the  effect  of  self-induction  :  The 
spark  terminals  of  a  Ruhmkorff  coil  (Fig.  22)  are  placed 

*  Lodge,  Modern  Views  of  Electricity,  p.  101. 


168 


WHAT  IS  ELECTRICITY! 


FIG.  22. 


in  a  vessel  from  which  the  air  can  be  exhausted.     This 
vessel  is  then  connected  with  an  air  pump.     In  the 

primary  circuit 
of  the  Ruhm- 
korff  coil  is 
placed  a  coil  of 
small  resistance. 
When  the  ex- 
haustion of  the 
vessel  is  pushed 
to  a  certain 
amount  —  about 
fourteen  inches 
of  mercury  pressure  in  the  vessel — the  insertion  of  a 
bundle  of  iron  wires  in  the  coil  in  the  primary  circuit 
completely  stops  the  formation  of  the  spark.  Work  is 
done  in  establishing  lines  of  force  in  the  medium  about 
the  coil,  and  less  work  can  be  done  in  producing  the 
spark.  If,  now,  the  exhaustion  is  pushed  farther,  the 
white  crackling  spark  is  replaced  by  a  reddish-purple 
glow,  and  now  the  increase  of  self-induction  produced 
by  inserting  the  bundle  of  iron  wires  no  longer  pro- 
duces any  visible  effect 
on  the  spark.  The  re- 
sistance of  the  rarefied 
air  has  become  less,  and 
the  change  in  the  self- 
induction  of  the  circuit 
is  not  sufficient  to  mod- 
ify the  transformation 
of  energy  in  the  trans- 
former. The  phenomenon  of  self-induction  can  be  illus- 
trated in  a  striking  manner  by  placing  a  coil  of  a  num- 
ber of  turns  but  of  comparatively  small  resistance  to- 


FIG.  23. 


SELF-INDUCTION.  169 

gether  with  an  electric  light  on  an  alternating-current 
circuit.  When  an  iron  core  is  thrust  into  the  coil  the 
electric  light  (Fig.  23)  is  greatly  changed  in  brilliancy. 
This  change  is  due  to  the  increased  self-induction  of  the 
circuit.  The  change  in  the  flow  of  induction  through 
the  coil  has  been  greatly  increased,  and  this  change  gives 
place  to  an  electro-motive  force  in  the  wire  of  the  coil 
which  is  opposed  to  the  electro-motive  force  of  the 
alternating-current  machine  which  is  feeding  the  cir- 
cuit. 

Work  is  required  to  establish  the  lines  of  flow  of 
induction  through  the  iron,  and  also  to  withdraw  them. 
The  phenomenon  of  self-induction  bears  greatly  upon 
the  question  of  lightning  rods  and  the  protection  of 
buildings  from  lightning. 
To  illustrate  this,  sup- 
pose that  we  should  dis- 
charge a  Leyden  jar 
(Fig.  24)  by  means  of  a 

wire    bent    into   a  long 

i  -j  j      -J?  FlG-  24- 

loop  and  provided  with 

two  points,  A  and  B,  which  can  be  brought  near  to- 
gether. It  will  be  found  that  when  the  spark  occurs 
between  C  and  D  that  a  spark  will  also  occur  between 
A  and  B.  The  electrical  current  prefers  to  pass  be- 
tween A  and  B,  and  thus  to  avoid  the  work  of  putting 
lines  of  force  around  the  loop  M.  The  reason  that 
lightning  takes  the  shortest  path  is  not  that  this  path 
has  the  least  resistance,  but  that  it  has  the  least  induc- 
tance. The  resistance  of  the  air  between  A  and  B  is 
far  more  than  that  of  the  copper  loop  M.  We  can 
therefore  conceive  of  a  building  being  shattered  by 
lightning  which  is  provided  with  lightning  rods,  if  it 
requires  less  work  to  pass  through  the  building  to  the 


170  WHAT  IS  ELECTRICITY! 

ground  than  to  overcome  the  self-induction  of  the 
lightning  rod.  When  we  read  Franklin's  observations 
on  the  utility  of  lightning  rods,  we  perceive  that  he 
had  no  conception  of  the  phenomenon  of  self-induction. 
To  his  mind  the  electrical  fluid  took  the  path  of  the 
good  conductor  in  all  cases.  It  may  have  been  that 
sceptical  ones  of  his  day  treasured  up  instances  of  light- 
ning leaving  good  conductors  in  a  most  unaccountable 
manner,  and  were  believers  in  an  opposing  effect,  which 
we  now  call  inductance. 

Prof.  John  Winthrop  wrote  to  Franklin  as  follows : 

CAMBRIDGE,  January  6, 1768. 

"  I  have  read  in  the  Philosophical  Transactions  the 
account  of  the  effects  of  lightning  on  St.  Bride's  steeple. 
It  is  amazing  to  me  that,  after  the  full  demonstration 
you  had  given  of  the  identity  of  lightning  and  electrici- 
ty and  the  power  of  metalline  conductors,  they  should 
ever  think  of  repairing  that  steeple  without  such  con- 
ductors. How  astonishing  is  the  force  of  prejudice,  even 
in  an  age  of  so  much  knowledge  and  free  inquiry ! "  * 

*  Sparks,  Works  of  Benjamin  Franklin,  vol.  v,  p.  419. 


CHAPTER  XIV. 

THE    LEYDEN    JAB. 

IN  our  further  study  of  the  transformation  of  en- 
ergy self-induction  plays  a  most  important  role.  The 
coefficient  of  induction  multiplied  by  one  half  of  the 
square  of  the  current  represents  the  energy  which  is 
stored  up  in  the  medium  surrounding  the  wire  carrying 
the  current.  Any  change  in  the  value  of  the  current 
produces  a  change  in  the  amount  of  this  energy.  An- 
other factor  which  is  never  absent  on  electrical  circuits 
is  what  is  called  capacity.  A  Leyden  jar  has  capacity. 
We  ordinarily  say  that  a  certain  amount  of  electricity  can 
be  stored  up  in  the  jar.  The  larger  the  jar,  the  greater 
the  capacity.  The  Atlantic  cable  has  a  large  capacity. 
It  is  a  long,  cylindrical  Leyden  jar;  the  conducting 
wire  forms  the  inner  coating  and  the  water  the  outer 
coating.  In  the  case  of  a  thunderstorm,  the  upper  layer 
of  clouds  forms  one  coating  of  a  condenser  and  the  sur- 
face of  the  earth  the  other,  while  the  air  between  takes 
the  place  of  the  glass  in  an  ordinary  Leyden  jar  or  of 
the  gutta-percha  of  the  Atlantic  cable.  The  telegraph 
wires  strung  on  poles  also  possess  capacity.  The  wires 
form  one  surface  of  the  condenser  and  the  ground  the 
other. 

The  insulator  between  two  charged  surfaces  of 
metal  is  called  the  dielectric,  and  modern  inquiry  is 

171 


172  .     WHAT  IS  ELECTRICITY! 

largely  directed  to  the  study  of  what  goes  on  in  the 
dielectrics  under  rapidly  alternating  charges  on  the 
metals.  There  is  no  doubt  that  a  stress  occurs  in  the 
dielectric  under  heavy  charges,  for  the  glass  walls  of 
Leyden  jars  are  often  broken,  and  we  see  how  the  air 
is  cracked,  so  to  speak,  by  discharges  of  lightning. 
The  ordinary  Edison  lamp  which  is  used  to  light  our 
houses,  consisting  of  a  glass  bulb  inclosing  a  carbon 
filament,  has  considerable  capacity.  It  is  a  small  Ley- 
den  jar.  If  one  holds  the  bulb  in  one's  hand  and  pre- 
sents the  brass  base  of  the  lamp  to  the  conductor  of  an 
electrical  machine,  after  a  moment  of  charging  one  can 
obtain  a  shock  by  touching  the  brass  base  of  the  lamp 
with  one  hand  while  the  bulb  is  held  in  the  other. 

When  we  survey  the  path  we  have  followed  in 
studying  the  electric  current  and  the  various  transfor- 
mations of  energy  which  are  manifested  by  the  rate  of 
change  of  electro-magnetic  induction,  we  perceive  that 
our  attention  has  been  directed  mainly  to  the  electrical 
manifestations  on  closed  metallic  circuits.  Indeed,  to 
the  ordinary  mind  a  wire  seems  to  be  the  essential  part 
of  an  electric  circuit.  Thus,  when  we  discharge  a  Ley- 
den  jar  by  connecting  the  outer  coating  to  the  inner 
coating  by  a  wire,  we  are  apt  to  fix  our  minds  upon 
this  wire  as  the  seat  of  a  momentary  electric  current, 
the  energy  of  which  is  manifested  by  the  spark  which 
results  when  the  jar  is  discharged.  We  know  that 
there  is  a  current  in  the  wire  when  the  jar  is  discharged, 
for  it  will  melt  a  wire  and  decompose  water. 

Faraday,  in  1832,  made  experiments  on  the  quantity 
of  electricity  yielded  by  the  discharge  of  a  Leyden  jar ; 
and  stated  his  results  as  follows :  "  The  decomposition 
of  a  single  grain  of  water  requires  800,000  discharges 
of  a  large  Leyden  battery.  This,  if  concentrated  in  a 


THE  LEYDEN  JAR.  173 

single  discharge,  would  be  equal  to  a  very  great  flash 
of  lightning,  while  the  chemical  action  of  a  single  grain 
of  water  on  four  grains  of  zinc  would  yield  electricity 
equal  in  quantity  to  a  powerful  thunderstorm."  * 

The  prevailing  impression  is  that  more  electricity 
can  be  obtained  from  a  percussion  cap  filled  with  moist 
salt  sand  in  which  a  piece  of  zinc  wire  is  immersed,  not 
touching  the  copper  of  the  cap,  than  from  a  discharge 
of  lightning.  One  can  send  a  signal  across  the  Atlantic 
cable  with  such  a  minute  battery ;  but  it  is  said  one  can 
not  do  this  with  a  spark  from  a  Leyden  jar.  This  last 
assertion,  however,  is  a  mistake.  It  can  be  -done  by 
means  of  the  spark  from  a  Leyden  jar.  All  that  is 
necessary  is  to  properly  transform  this  spark  in  the  fol- 
lowing manner : 

Coat  any  large  thin  glass  vessel  on  the  outside  with 
tin  foil  and  fill  the  vessel  with  water.  Now  connect 
the  outside  tin-foil  surface  with  one  conductor  of  an 
electrical  machine  and  the  water  inside  the  vessel  with 
the  other  conductor  of  the  machine.  After  a  few  turns 
of  the  machine  the  Leyden  jar  becomes  charged  ;  and 
if  the  water  on  the  inside  is  connected  by  means  of  a 
wire  with  the  tin  foil  on  the  outside,  a  spark  passes 
when  the  end  of  the  wire  is  brought  near  the  tin  foil. 

Instead,  however,  of  allowing  the  spark  to  dissipate 
itself  in  light  and  noise,  let  us  connect  the  tin  foil  with 
one  end  of  a  bobbin  of  well-insulated  fine  wire  of  a 
thousand  feet  or  more  in  length,  but  wound  compactly 
on  a  hollow  bobbin.  In  the  centre  of  this  bobbin,  en- 
tirely disconnected  and  insulated  from  the  fine-wire 
bobbin,  we  will  place  another  coil  of  coarse  wire  five  or 
six  feet  in  length  wound  once  around  a  bundle  of  iron 

*  Dr.  Bence  Jones,  Life  of  Faraday. 


174:  WHAT  IS  ELECTRICITY! 

wire.  Across  the  ends  of  this  coarse  wire  we  will  place 
a  small  incandescent  lamp  of  from  five  to  six  candle 
power.  Now,  if  the  other  end  of  the  fine-wire  bobbin 
is  brought  near  the  inside  of  the  Leyden  jar,  a  spark 
jumps  and  is  dissipated  through  the  thousand  feet  of 
the  fine  wire.  The  little  lamp  connecting  the  ends  of 
the  coarse  coil  lights  up  for  an  instant.  If,  instead  of 
the  lamp,  two  platinum  wires  are  placed  in  acidulated 
water,  and  are  connected  with  the  ends  of  the  coarse 
coil,  a  quantity  of  babbles  of  oxygen  and  hydrogen  gas 
is  given  off  from  each  of  the  platinum  wires.  The 
water  is  decomposed,  just  as  it  is  by  two  or  three  strong 
chemical  or  voltaic  cells. 

We  see,  therefore,  that  it  is  merely  a  question  of 
transformation.  An  electric  spark  can  do  all  that  a 
battery  or  a  dynamo  can  do.  It  works,  however,  for  a 
very  short  interval  of  time.  It  has  the  characteristic 
of  brilliancy  but  not  of  persistence.  A  simple  calcula- 
tion will  enable  us  to  form  an  idea  of  the  horse  power 
in  a  spark  from  a  Leyden  jar  of  about  a  gallon  capaci- 
ty, the  glass  of  which  is  about  one  sixteenth  of  an  inch 
thick  and  which  is  charged  so  that  it  will  give  a  spark 
of  about  two  inches  long. 

Such  a  spark  discharged  through  our  bobbin  con- 
taining about  a  thousand  feet  of  wire  will  light  up 
brilliantly  a  six-candle-power  lamp  connected  with  the 
coarse-wire  bobbin  which  occupies  the  centre  of  the 
fine-wire  bobbin.  Now  we  know  from  accurate  experi- 
ments that  the  spark  lasts  a  few  hundred  thousandths 
of  a  second — it  may  be  three  hundred  thousandths. 
We  know  also  a  horse  power  would  light  from  thirty 
to  forty  of  our  little  lamps.  If  there  were  no  loss  in 
transforming  the  spark,  the  spark  would  be  equal  to 
one  thirtieth  of  a  horse  power  acting  for  three  hundred 


THE  LEYDEN  JAR.  175 

thousandths  of  a  second  ;  but  there  is  a  loss  in  transfor- 
mation of  nearly  fifty  per  cent,  so  the  horse  power  in 
our  spark  is  twice  what  we  have  supposed — two  thir- 
tieths or  one  fifteenth  of  a  horse  power. 

In  a  subsequent  paper  I  shall  show  that  we  have 
reasons  for  believing  that  the  energy  of  an  electric 
spark  an  inch  in  length  amounts  to  thirty  or  forty 
horse  power ;  for  waves  are  sent  out  in  the  ether  in  all 
directions,  and  these  waves  are  of  great  energy.  At 
present,  however,  we  are  concerned  only  with  a  direct 
transformation  of  an  electric  spark  into  horse  power 
which  can  be  directly  measured. 

Kow,  if  a  spark  two  inches  in  length  from  a  gallon 
Ley  den  jar  is  equivalent  to  one  fifteenth  of  a  horse 
power,  what  must  be  the  horse  power  in  discharges  of 
lightning  which  are  many  hundred  feet  in  length  ?  We 
have  all  heard  the  bells  of  telephone  apparatus  ring 
violently  at  each  discharge  of  lightning,  and,  on  timing 
the  interval  between  the  flash  and  the  thunder,  we  find, 
knowing  that  sound  travels  about  a  thousand  feet  per 
second,  that  the  discharge  must  have  occurred  a  mile 
away.  We  should  find,  on  making  the  necessary  calcu- 
lation, that  it  would  take  some  hundreds  of  horse  power 
to  produce  this  electrical  effect  from  the  distance  of  a 
mile. 

Incandescent  lamps  also  often  blink  at  each  discharge 
of  lightning  from  a  storm  centre  at  least  a  mile  away. 
When  the  discharges  occur  within  a  thousand  feet  the 
lamps  may  be  nearly  extinguished  for  an  instant ; 
therefore  the  lightning,  even  at  a  distance  of  a  thou- 
sand feet,  holds  in  check  it  may  be  a  thirty-horse-power 
steam-engine  which  is  turning  the  dynamo  machine  and 
supplying  the  lights.  I  have  no  doubt  that  a  discharge 
of  lightning  five  hundred  feet  long,  if  properly  directed 


176  WHAT  IS  ELECTRICITY? 

and  controlled,  could  light  for  an  instant  a  thousand 
Edison  lights. 

It  seems  at  times  as  if  the  bolts  of  lightning  grow 
envious  of  the  great  webs  of  wire  which  have  been 
spread  over  our  cities,  and  delight  to  exhibit  their  horse 
power  by  entering  upon  electric-light  circuits,  showing 
the  dynamos  how  to  burn  out  wires  and  set  fire  to 
buildings.  Indeed,  one  of  the  most  serious  concerns  of 
the  practical  electrician  is  to  devise  methods  of  pre- 
venting lightning  from  breaking  and  entering.  There 
is  a  popular  superstition  that  the  multiplication  of  elec- 
tric circuits  in  our  cities  and  towns  has  driven  off  thun- 
derstorms, but  there  is  no  proof  that  such  is  the  case. 
The  lightning  is  still  an  unwelcome  visitor,  and  comes 
at  the  most  unexpected  times.  I  have  no  doubt,  how- 
ever, that  the  multiplication  of  wires  is  to  a  certain 
extent  a  safeguard  against  the  exhibition  of  the  horse 
power  of  lightning  by  its  destruction  of  chimney  tops, 
rending  of  trees,  and  even  the  killing  of  human  be- 
ings ;  for  the  multitude  of  wires  distributes  the  elec- 
trical charge,  and  it  finds  a  quick  passage  to  the  ground 
in  many  directions. 

I  have  already  said  that  the  practice  of  combin- 
ing gas  fixtures  and  electric-light  circuits  so  that  one 
can  use  gas  or  electricity  is  fraught  with  some  dan- 
ger. The  electric  wires  are  often  led  along  the  gas 
pipes,  and  if  lightning  should  succeed  in  following  the 
electric  wires  into  the  house,  it  would  naturally  jump 
to  the  gas  pipes  and  seek  the  ground.  If  there 
should  be  a  gas  leak,  even  from  a  minute  pin  hole,  it 
might  be  lighted.  If  the  lightning  does  not  enter  the 
house  by  the  wires,  it  is  possible  that  a  heavy  discharge 
may  cause  sparks  between  the  electric  wires  and  the 
gas  pipes  by  induction.  I  knew  of  an  instance  where 


THE  LEYDEN  JAR.  177 

a  spark  between  the  electric-light  wires  and  the  gas 
pipes  ignited  the  gas  which  streamed  from  a  minute 
hole  in  the  pipe.  If  the  jet  had  not  been  noticed  in 
time  the  building  would  surely  have  been  set  on  fire. 
The  building  was  provided  with  lightning  fuses ;  never- 
theless, the  minute  sparks  were  caused  by  a  lightning 
discharge.  In  building  a  new  house  one  should  be  care- 
ful to  keep  the  electric-wire  circuit  away  from  the  gas 
pipes.  The  practical  electrician  and  the  theoretical 
plumber  would  doubtless  call  this  a  scientific  man's 
superstition,  but  a  long  study  of  electrical  sparks  has 
made  me  respect  their  wide  and  varied  manifestations 
of  energy. 

The  more  that  one  knows  about  the  horse  power  in 
lightning  the  more  one  wonders  at  the  temerity  of  Ben- 
jamin Franklin  in  drawing  lightning  from  the  clouds. 
He  evidently  regarded  it  as  a  lambent  ethereal  flame, 
capable,  it  is  true,  of  giving  disagreeable  shocks ;  he  knew 
its  power  in  rending  trees  and  hi  setting  fire  to  build- 
ings, yet  he  could  not  have  had  a  realizing  sense  of  its 
horse  power.  No  one  to-day  would  be  willing  to  re- 
peat Franklin's  experiment  in  the  manner  that  he  per- 
formed it.  One  could,  it  is  true,  lead  the  wet  string 
into  a  lake  or  pond  and  hold  the  string  with  rubber 
gloves.  Most  of  us,  even  so,  would  prefer  to  be  interest- 
ed spectators  rather  than  participants  in  the  experiments. 

Looked  at  from  another  point  of  view,  it  will  be 
seen  that  the  force  required  to  rend  the  air — to  bore  a 
hole,  so  to  speak,  through  it  as  lightning  does,  to  crack 
it  as  if  it  were  a  piece  of  glass — must  be  enormous. 

Nothing,  to  my  mind,  so  strongly  illustrates  the  dif- 
ference in  the  intellectual  standpoint  of  the  ancients 
and  that  of  the  moderns  in  regard  to  science  as  such  a 
discussion  as  this  upon  the  horse  power  of  lightning. 


178  WHAT  IS  ELECTRICITY? 

Philosophers  to-day  do  not  speculate  about  the  primal 
sources  of  lightning,  but  set  themselves  to  work  to 
study  the  transformations  of  electricity,  with  a  large 
hope  that  they  can  greatly  increase  our  knowledge  of 
such  transformations  and  with  very  little  hope  that 
they  can  ascertain  what  electricity  really  is.  Fancy 
Faraday's  delight,  could  he  have  seen  the  working  of 
the  modern  transformer,  the  fine-wire  bobbin  inclosing 
the  coarse  coil  with  its  bundle  of  iron  wires !  imagine 
the  immense  field  of  the  practical  applications  of  elec- 
tricity which  would  immediately  have  opened  to  his 
vision  !  Cities  are  now  lighted  by  its  means,  and  it  is 
proposed  to  transmit  to  great  distances  by  the  trans- 
former the  power  of  Niagara  Falls. 

Seeing  thus  the  possibility  of  transforming  the  elec- 
tric spark  into  working  horse  power,  so  to  speak,  a 
question  more  or  less  curious  intrudes  itself  upon  one's 
mind.  Is  it  not  possible  to  make  a  practical  use  of  the 
electrical  machines  which  have  since  the  time  of  Benja- 
min Franklin  played  their  part  upon  the  lecture  table 
of  professors  of  physics  ?  Unmanageable  servants  they 
are  often,  inopportunely  festive,  brilliant ;  but,  on  the 
whole,  not  to  be  depended  upon  in  all  weathers.  The 
professor's  Buhmkorff  coil  has  taken  its  place  in  prac- 
tical life  as  a  transformer  such  as  we  have  described 
in  this  article,  but  not  to  transform  lightning.  It  is 
used  in  every  telephone  transmitter  in  the  land,  and 
is  employed,  as  we  have  said,  in  lighting  cities  and 
in  transmitting  power  long  distances.  The  dynamo 
machine  also  has  sprung  into  giant  shape  from  its 
lecture-room  models.  Why  should  not  the  electrical 
machine  also  have  its  practical  development,  since  we 
have  seen  that  its  sparks  can  be  transformed  into  horse 
power  ? 


THE  LEYDEN  JAR.  179 

It  apparently  requires  very  little  power  to  turn  the 
disks  of  the  Holtz  machine  to  produce  sparks  which 
when  transformed  will  light  up  for  an  instant  an  eight- 
or  ten-candle-power  lamp.  Why  could  not  one  ar- 
range a  number  of  electrical  machines  in  such  a  man- 
ner that,  turned  by  a  common  shaft,  they  might  charge 
and  discharge  Leyden  jars  continuously,  and  thus,  by 
means  of  the  transformer  we  have  described,  produce 
light  ?  It  is,  indeed,  conceivable  that  a  great  number 
of  large  electrical  machines  of  the  late  improved  types 
could  be  driven  by  steam  power  or  water  power,  and 
their  charges  so  accumulated  in  suitable  Leyden  jars  or 
condensers  that  a  large  building  could  be  illuminated. 
Since  it  requires  time  to  charge  Leyden  jars  to  their 
full  capacity,  a  great  number  of  large  electrical  ma- 
chines would  be  required,  and  the  intervals  between 
successive  discharges  of  the  jars  could  not  be  less  than 
one  sixteenth  of  a  second,  in  order  that  the  instantane- 
ous lighting  of  the  lamps  should  remain  on  the  eye  and 
seem  to  be  continuous. 

This  endeavour  to  imitate  the  action  of  the  ordinary 
dynamo  machine  by  coupling  electrical  machines  to- 
gether is  much  more  difficult  at  present  than  to  pro- 
ceed in  the  inverse  order  and  to  imitate  the  action  of 
the  electrical  machine  by  means  of  the  dynamo  ma- 
chine. If  we  send  a  powerful  dynamo  current  to  and 
fro  through  the  coarse  coil  of  the  transformer  which 
we  have  used  to  exhibit  the  horse  power  of  an  electric 
spark,  we  can  readily  obtain  sparks  of  several  feet  in 
length  from  the  ends  of  the  fine  wire  of  the  outer 
bobbin.  By  a  suitable  transformation  in  the  coil  we 
can  cause  an  exhausted  globe  to  become  luminous  by 
pointing  the  finger  at  it ;  we  can  make  a  lamp  glow 
without  leading  wires  when  it  is  placed  anywhere  be- 
13 


180  WHAT  IS  ELECTRICITY? 

tween  the  walls  of  a  room  which  are  connected  with 
the  ends  of  our  transformer.  The  entire  room  can  be 
filled  with  lines  of  electric  force,  but  it  will  be  at  the 
expense  of  a  large  amount  of  horse  power. 

To  make  one  lamp  glow  without  leading  wires  when 
it  is  placed  anywhere  in  a  small  room,  requires  at  pres- 
ent the  expenditure  of  at  least  twenty-five  to  thirty 
horse  power.  This  does  not  seem  to  be  the  light  of  the 
future.  A  large-sized  electrical  machine  can  even  now 
compete  with  the  dynamo  machine  in  experiments  of 
this  sort  in  producing  phosphorescent  glow  lamps. 
The  dynamo  machine,  it  is  true,  can  imitate  all  the 
effects  of  an  electrical  machine — its  long  spark,  its 
phosphorescent  glow  lamps — but  it  does  this  with  a 
great  expenditure  of  horse  power.  On  the  other  hand, 
if  the  electrical  machine  should  endeavour  to  perform 
the  work  of  a  dynamo  machine  hi  lighting  incandes- 
cent lamps,  it  would  be  also  at  a  great  expense  of  horse 
power. 

The  practical  electrician  of  to-day  has  comparatively 
little  use  for  the  Leyden  jar,  or  for  the  modifications 
which  are  called  condensers,  except  in  the  case  of  sub- 
marine telegraphy.  The  cable  itself,  we  have  said,  is  a 
great  cylindrical  Leyden  jar.  We  can  not  telegraph 
along  the  central  core  of  this  cable  by  the  same  means 
which  we  employ  on  land  lines — that  is,  by  connecting 
a  battery  direct  to  this  core  and  using  electro-magnets 
or  Morse  sounders.  The  cable  charges  under  the  elec- 
tro-motive force  of  the  battery,  just  as  a  Leyden  jar 
charges  under  the  difference  of  potential  of  an  electrical 
machine.  The  line  therefore  becomes  clogged,  so  to 
speak,  with  the  charges  sent  into  it  by  the  different 
impulses  from  the  battery,  and  the  result  is  a  confusion 
of  signals.  The  capacity  of  the  cable  is  so  great  that 


THE  LEYDEN  JAR.  181 

time  is  necessary  both  to  charge  and  to  discharge  it. 
This  confusion  is  made  very  evident  if  we  attempt  to 
use  a  telephone  and  speak  over  the  cahle.  At  a  dis- 
tance of  about  fifty  miles  nothing  but  a  murmur  is 
heard ;  the  characteristics  of  the  human  voice  are  com- 
pletely obliterated  by  the  capacity  of  the  cable.  In 
order  to  speak  under  the  ocean,  some  way  must  be 
found  of  neutralizing  this  capacity.  To  the  electrician 
whose  work  lies  in  the  field  of  telephony  the  subject  of 
the  Leyden  jar  therefore  is  of  the  utmost  importance. 
Meanwhile,  to  obviate  the  disturbing  effect  of  the  ca- 
pacity of  the  cable  we  do  not  connect  our  battery  direct 
to  the  cable,  but,  having  first  charged  a  condenser  by 
connecting  its  two  coatings  to  a  battery,  we  discharge 
the  condenser  into  the  cable  and  use  very  delicate  in- 
struments to  record  the  feeble  current  which  thus  trav- 
erses the  cable.  The  process  of  submarine  telegraphy 
is  thus  simply  one  of  discharging  a  Leyden  jar  through 
it.  A  comparatively  large  electro-motive  force  can 
thus  be  used  without  the  danger  of  heating  the  wire 
of  the  cable  and  without  giving  the  cable  too  great  a 
charge. 

In  the  submarine  cable,  therefore,  the  practical  elec- 
trician sees  the  importance  of  the  study  of  the  Leyden 
jar.  To  the  scientific  man  its  manifestations  have  be- 
come of  greater  importance  than  the  electrical  effects 
we  have  hitherto  studied  in  metallic  circuits,  for  he  is 
persuaded  that  by  the  study  of  the  electrical  phenomena 
in  the  dielectric — the  glass,  for  instance,  of  a  Leyden 
jar — we  shall  more  clearly  understand  the  relationship 
between  light,  heat,  and  electricity.  In  an  ordinary 
voltaic  cell  we  can  trace  the  complete  circuit  of  elec- 
trical phenomena.  The  electro-motive  force  arises  in 
some  mysterious  way  from  the  two  metals  of  the  bat- 


182  WHAT  IS  ELECTRICITY  f 

tery ;  a  current  results  from  this  force  and  the  chem- 
ical actions  in  the  cell.  This  current  can  be  traced  on 
the  wire  connecting  the  two  metals  of  the  cell  and  in 
the  liquid  of  the  cell.  There  is  no  part  of  this  circuit 
where  there  is  no  evidence  of  an  electric  current.  It  is 
present  in  the  outer  wire  which  rings  our  house  bell ; 
it  is  evident  in  the  liquid  of  the  cell.  In  the  case, 
however,  of  a  Leyden  jar  the  current  manifests  itself 
apparently  only  in  the  wire  connecting  the  outer  and 
inner  coating  of  the  jar.  The  glass  that  separates  these 
coatings  is  called  an  insulator ;  it  does  not  permit  a  cur- 
rent of  electricity  to  pass  through  it.  How,  then,  is 
the  electric  circuit  completed  ?  Are  there  no  electrical 
effects  in  the  glass  ?  The  great  theory  of  Maxwell — 
the  electro-magnetic  theory  of  light — assumes  that  there 
is  an  electrical  effect  in  the  dielectric  in  the  shape  of 
what  are  called  displacement  currents,  which  arise  when 
the  electrical  energy  which  is  stored  up  in  the  ether 
near  the  metallic  coatings  of  the  jar  is  made  to  fluctuate 
in  amount.  The  cycle  of  electrical  effects  manifested 
by  a  Leyden  jar  is  made  up,  therefore,  of  currents  of 
conduction  along  the  wires  connecting  the  coatings  and 
of  displacement  currents  in  the  glass.  The  study  of 
these  displacement  currents,  both  from  a  mathematical 
and  an  experimental  point  of  view,  is  now  of  the  utmost 
theoretical  importance. 

The  phenomena  of  the  spark  from  an  electrical 
machine  or  a  charged  Leyden  jar  differ  from  those 
exhibited  by  the  voltaic  arc  mainly  in  this  :  the  spark 
studied  by  Franklin  is  not  one  spark,  but  each  spark  is 
made  up  of  a  number  of  sparks  which  oscillate  to  and 
fro.  This  was  first  shown  by  Joseph  Henry,  and  it  is 
a  remarkable  fact,  the  importance  of  which  will  be 
seen  when  we  study  electrical  waves. 


THE   LEYDEN  JAR.  183 

When  we  obtain  a  great  difference  of  potential  be- 
tween the  carbon  terminals — for  instance,  when  a  spark 
from  a  Leyden  jar  jumps  between  them — we  perceive 
a  marked  difference  between  these  poles.  A  good  way 
to  study  these  differences  is  to  distribute  light  powder, 
like  lycopodium,  on  glass,  bring  wires  from  a  Ruhmkorff 
coil  to  opposite  points  on  the  glass,  and  break  the  cir- 
cuit of  the  primary.  The  light  dust  is  disturbed,  and  a 
figure  will  result  on  the  glass  due  to  the  state  of  electri- 
fication of  the  point  of  the  wire.  These  figures  are  dif- 
ferent at  the  positive  and  negative  terminals  of  the  wire, 
and  they  have  been  carefully  studied  by  Bezold  and  other 
physicists.  Another  interesting  way  to  study  these  fig- 
ures is  to  replace  the  glass  plate  and  the  dust  by  an  ordi- 
nary photographic  plate.  On  developing  the  plate  a 
photograph  is  obtained  which  shows  that  the  discharge 
at  either  the  negative  or  the  positive  pole  is  a  complex 
figure  made  up  of  both  the  discharge  peculiar  to  the 
negative  pole  and  that  peculiar  to  the  positive  pole. 
Now  it  may  be  asked,  How  do  we  know  that  the  figure 
is  a  combination  of  these  effects  ?  What  we  wish  to  show 
is,  that  the  discharge  from  a  Leyden  jar  is  a  to-and-fro 
discharge — that  it  is  oscillatory,  in  other  words.  In  or- 
der to  prove  this,  it  is  necessary  to  use  a  revolving  mir- 
ror to  study  the  discharge.  The  following  is  the  modi- 
fied form  of  Feddersen's  apparatus  which  I  have  used  in 
my  researches,  and  which  I  shall  often  have  occasion  to 
refer  to  in  what  follows :  A  little  concave  mirror  is 
mounted  on  the  armature  shaft  of  a  little  electric  mo- 
tor, E  (Fig.  25).  The  spark  gap  is  placed  just  above  a 
sensitive  plate,  P,  which  is  shielded  from  the  direct 
light  of  the  spark.  If  the  mirror,  M,  is  at  rest,  the  photo- 
graph obtained  by  reflection  at  P  is  simply  a  zigzag 
line.  When,  however,  the  mirror  revolves  very  swiftly 


184  WHAT  IS  ELECTRICITY? 

the  photograph  of  the  spark  is  drawn  out  into  a  band 
like  a  comet's  tail,  which  is  seen  to  be  made  up  of  a 
series  of  dots  of  alternate  degrees  of  brightness  (Plate  I, 
frontispiece).  The  darkest  dots  represent  the  stronger 
discharge  at  the  positive  terminal,  and  the  light  dots  the 
discharge  at  the  negative 
terminal.  The  discharge 
oscillates  to  and  fro  until  it 
dies  out.  What  seems  to 
the  eye,  therefore,  as  one 
spark  is  made  of  a  number. 
The  number  depends  upon 
the  length  of  wire  in  the 
FlG  25  circuit  between  the  two 

coatings  of  the  jar  and  the 

thickness  and  size  of  the  jar,  or,  in  other  words,  what 
is  termed  the  capacity  of  the  jar.  It  may  be  thought 
that  a  rough  manner  of  showing  the  oscillatory  nature 
of  the  discharge  of  a  Leyden  jar  would  be  to  perforate 
a  piece  of  writing  paper  by  the  discharge ;  for,  on  ex- 
amining the  hole  made  by  the  spark,  it  will  be  seen  to 
have  a  burr  on  both  sides  of  the  paper,  as  if  the  dis- 
charge had  passed  to  and  fro  through  it.  This  method, 
however,  is  an  erroneous  one,  for  if  the  discharge  is 
made  nonoscillatory  by  discharging  through  a  wet  string 
or  a  suitable  liquid  resistance,  the  burrs  are  also  ob- 
tained. The  phenomenon  is  therefore  due  to  a  species 
of  explosion  in  the  paper,  and  is  not  due  to  the  oscilla- 
tions of  the  spark  discharge. 

If  we  analyze  an  ordinary  spark  by  the  revolving 
mirror  we  discover  that  it  is  generally  oscillatory. 
When,  therefore,  people  imagine  that  they  can  tell 
which  way  a  lightning  discharge  passes,  whether  from 
the  clouds  to  the  earth,  or  from  the  earth  to  the  sky, 


THE  LEYDEN  JAR.  185 

they  must  reflect  upon  this  oscillatory  phenomenon,  and 
also  consider  that  the  interval  between  such  oscillations 
is  less  than  one  ten  millionth  of  a  second.  An  impres- 
sion remains  on  the  retina  about  one  sixteenth  of  a 
second,  and  the  human  eye,  therefore,  can  not  distin- 
guish direction  in  the  electric  spark. 

I  have  used  the  following  method  of  studying  the 
phenomena  at  the  poles  of  discharge :  The  terminals, 
between  which  the  spark  jumps,  consist  of  two  thermal 
junctions.  Immediately  after  the  discharge  occurs  the 
circuit  between  the  junctions  is  completed  through  a 
galvanometer  by  a  peculiar  key.  In  the  case  of  the 
oscillatory  discharge,  the  two  terminals  are  heated 
equally,  and  there  is  no  movement  of  the  galvanometer 
needle.  When,  however,  the  revolving  mirror  shows 
that  the  discharge  is  nonoscillatory — and  this  can  be 
accomplished  by  putting  in  a  suitable  liquid  resistance 
in  the  path  between  the  two  coatings  of  the  Leyden  jar 
— the  galvanometer  shows  that  one  junction  at  the  posi- 
tive terminal  is  more  heated  than  that  at  the  negative. 

In  1842,  Prof.  Henry,  in  speaking  of  what  was 
called  anomalous  magnetism,  which  was  observed  in 
the  case  of  needles  magnetized  by  discharges  from 
Leyden  jars — these  needles  often  exhibiting  a  magnetic 
condition  opposite  to  that  which  should  result  from  a 
current  in  a  definite  direction — says : 

"  This  anomaly,  which  has  remained  so  long  unex- 
plained, and  which  at  first  sight  appears  at  variance 
with  all  our  theoretical  ideas  of  the  connection  of  elec- 
tricity and  magnetism,  was,  after  considerable  study, 
satisfactorily  referred  by  the  author  to  an  action  of  the 
discharge  of  the  Leyden  jar  which  had  never  before 
been  recognised.  The  discharge,  whatever  may  be  its 
nature,  is  not  correctly  represented  (employing  for  sim- 


186  WHAT  IS  ELECTRICITY? 

plicity  the  theory  of  Franklin)  by  the  single  transfer  of 
an  imponderable  fluid  from  one  side  of  the  jar  to  the 
other ;  the  phenomenon  requires  us  to  admit  the  exist- 
ence of  a  principal  discharge  in  one  direction,  and  then 
several  reflex  actions  backward  and  forward,  each  more 
feeble  than  the  preceding,  until  the  equilibrium  is  ob- 
tained. All  the  facts  are  shown  to  be  in  accordance 
with  this  hypothesis,  and  a  ready  explanation  is  afforded 
by  it  of  a  number  of  phenomena  which  are  to  be  found 
in  the  older  works  on  electricity,  but  which  have  until 
this  time  remained  unexplained."  * 


*  Scientific  Writings  of  Joseph  Henry,  vol.  i,  p.  201,  Smithso- 
nian Institution,  Washington. 


CHAPTER  XV. 

STEP-UP   TRANSFORMERS. 

THE  range  of  transformations  of  energy  which  the 
Ruhmkorff  coil  exhibits  is  by  no  means  exhausted. 
When  we  succeed  in  producing  from  a  battery  a  spark 
similar  to  that  generated  by  an  electrical  machine,  we 
have,  by  the  use  of  a  fine-wire  coil  wound  upon  a  coarse- 
wire  coil,  exalted  the  electro-motive  force  of  three  or 
four  voltaic  cells — it  may  be  eight  volts — to  perhaps 
twenty  to  thirty  thousand.  Starting  from  this  great 
difference  of  potential,  is  it  possible  to  treat  the  Ruhm- 
korff coil  as  a  battery,  and  to  still  further  exalt  its  dif- 
ference of  potential  ?  This  has  been  done  by  Prof. 
Elihu  Thomson,  and  also  by  Tesla,  and  their  experi- 
ments are  most  brilliant  in  the  subject  of  the  trans- 
formations of  energy. 

Prof.  Thomson  has  succeeded  in  producing  sparks 
five  feet  long,  and  states  his  belief  that  sparks  twenty 
feet  in  length  could  be  obtained  by  an  extension  of  the 
method  which  he  employed.  This  method  was  as  fol- 
lows :  An  open  single-layer  coil  of  coarse  wire  with 
about  ten  turns  constituted  the  new  primary.  Upon 
this  was  wound  about  three  hundred  turns  of  fine  wire. 
This  latter  coil  constituted  the  new  secondary.  Both 
coils  were  immersed  in  oil.  On  passing  through  this  new 
primary  coil  sparks  from  a  Ruhmkorif  coil  very  long 


188  WHAT  IS  ELECTRICITY  I 

sparks  can  be  obtained  from  the  secondary.  This  ar- 
rangement constitutes  a  species  of  double  Ruhmkorff 
coil,  or  two  step-up  transformers.  Instead  of  using  a 
battery  to  excite  the  first  Ruhmkorff  coil,  an  alternating- 
current  dynamo  is  employed.  Leyden  jars  are  con- 
nected to  the  terminals  of  the  first  Ruhmkorfl:  coil,  and 
these  are  rapidly  charged  by  the  dynamo.  A  spark 
gap  is  interposed  to  the  circuit  between  the  first  Ruhm- 
korff  and  the  second,  and  the  resulting  spark  is  blown 
out  by  a  jet  of  air  under  high  pressure.  The  second 
Ruhmkorif  gives  sparks  of  the  extraordinary  length  of 
five  feet  or  more.  This  length  of  spark  far  exceeds 
that  given  by  the  most  powerful  electrical  machine.  If 

a  plate  of  glass  is 
interposed  between 
the  terminals  of  the 
second  coil,  its  sur- 
face is  covered  with 
brilliant  ramifica- 
tions of  violet-col- 
oured sparks. 

To  heighten  the 
FIG.  26. 

results,     as     above 

mentioned,  the  spark  produced  in  the  secondary  of  the 
first  Ruhmkorif  at  M  N  (Fig.  26)  is  constantly  blown  out 
by  a  strong  blast  of  air.  This  blast  serves  the  function 
of  the  break  in  the  primary  at  P.  A  very  high  electro- 
motive force  can  thus  be  obtained  between  C  and  D. 

I  have  said  that  we  have  here  two  step-up  trans- 
formers in  the  shape  of  two  Ruhmkorff  coils.  A 
Leyden  jar  is  interposed,  as  shown  in  the  figure,  in  or- 
der to  increase  the  quantity  of  electricity  which  is  dis- 
charged through  the  primary  of  the  second  trans- 
former, and  also  to  act  as  a  second  alternating  machine. 


STEP-UP  TRANSFORMERS.  189 

With  this  arrangement  of  the  apparatus  very  strong 
insulation  is  needed,  for  the  entire  line  is  charged  with 
electricity  of  high  tension.  The  leading  wires  are  lumi- 
nous in  the  dark,  from  the  brushlike  discharges  which 
are  given  off  in  every  little  break  in  the  insulation  of 
the  wires.  It  must  be  remembered  that  the  lines  of 
force  endeavour  to  leave  the  positive  terminal  of  the 
Ruhmkorff  coil  at  any  point  which  offers  a  shorter  pas- 
sage to  the  negative  terminal.  In  one  sense,  therefore, 
one  should  not  be  astonished  to  find  that  an  exhausted 
globe  will  become  luminous  when  it  is  attached  to 
merely  one  terminal  of  the  Ruhmkorff  coil,  for  the 
walls  of  the  room  and  the  floor  may  constitute  the  other 
terminal  of  the  coil,  and  the  lines  of  force,  stretching 
out  and  pervading  the  space  in  the  room,  converge  on 
matter  which  affords  in  any  way  the  easiest  passage. 
Thus  the  forefinger  glows  when  presented  to  either 
terminal  of  the  coil.  The  lines  of  force  find  on  the 
human  body  this  short  passage.  When  the  electro- 
motive force  or  electrical  intensity  is  greatly  enhanced 
the  tendency  of  the  lines  of  force  to  manifest  them- 
selves through  the  space  inclosed  in  any  ordinary  room 
is  greatly  increased.  At  the  same  time  the  to-and-fro 
currents  or  electrical  oscillations  on  the  leading  wires 
tend  to  confine  themselves  to  the  surface  of  these  wires. 
This  can  be  shown  in  a  popular  manner  by  connecting 
the  terminals  C  and  D  (Fig.  26)  by  a  thick  copper  rod, 
and,  holding  one  terminal  of  an  ordinary  incandescent 
lamp  in  one  hand  (Fig.  27),  touch  the  copper  loop  with 
the  other  terminal  and  also  grasp  the  loop  with  the 
other  hand.  Only  a  slight  shock  is  felt,  and  the  cur- 
rents passing  over  the  surface  of  the  human  body  raise 
the  carbon  filament  of  the  lamp  to  a  brilliant  incan- 
descence. The  surface  of  the  body  is  greater  than  that 


190  WHAT   IS  ELECTRICITY! 

of  the  copper  loop,  and  the  to-and-fro  currents  are  not 
compelled  to  confine  themselves  to  a  thin  film  of  cop- 
per constituting  the  surface  of  the  copper.  When 
steady  currents  are  passed  through  the  loop,  they  are 
not  confined  to  the  outer  layer  of  the  copper,  and  find 
an  easier  passage  through  the  section  of  the  copper 
loop  than  through  the  human  body.  With  steady  cur- 
rents it  is  impossible  to  light  the  lamp  in  the  above 


FIG.  27. 

manner  through  the  human  body.  The  effect  of  in- 
creasing the  frequency  of  to-and-fro  currents  of  elec- 
tricity is  thus  to  drive  them  to  the  surface  of  metallic 
conductors.  When  the  frequency  or  rapidity  of  vibra- 
tion is  enormous,  a  rod  of  copper  may  not  afford  any 
better  passage  than  a  rod  of  glass.  Hertz,  in  one  of 
his  papers,  points  out  that  our  present  nomenclature  is 
limited,  and  only  applies  to  the  special  cases  of  steady 
currents.  With  enormously  rapid  to-and-fro  currents, 
a  piece  of  copper  acts  like  an  insulator  and  prevents  any 
to-and-fro  currents  from  passing  through  it,  whereas  a 
piece  of  glass  transmits  them  unimpaired.  A  thick 
disk  of  copper  properly  placed  between  the  two  coils 


STEP-UP  TRANSFORMERS.  191 

of  a  step-up  transformer  can  completely  stop  the  elec- 
trical oscillations  from  reaching  a  lamp  connected  with 
the  secondary  coil  of  the  transformer,  whereas  a  plate 
of  glass  allows  them  to  pass  on  unimpeded.  Glass, 
therefore,  is  a  better  conductor  for  electrical  oscillations 
of  high  frequency  than  copper.  We  are  thus  approach- 
ing the  behaviour  of  light  to  these  two  substances. 

Is  it  not  possible,  therefore,  by  enormously  increas- 
ing the  frequency  of  electrical  oscillations,  to  drive  them 
completely  off  metallic  conductors  and  compel  them  to 
be  propagated  through  the  ether  of  space  ?  If  we  could 
do  this,  and  if  our  oscillation  should  meet  metallic  con- 
ductors, the'ir  energy  would  decay  or  be  absorbed  in 
such  conductors,  just  as  light  waves  are  absorbed  by 
nonconductors  of  light. 

The  latter  supposition  leads  us  again  to  Prof.  Poynt- 
ing's  view  of  the  decay  of  electro-magnetic  radiations 
which,  proceeding  from  the  sun,  pervade  all  space 
about  us.  This  decay  produces  the  phenomenon  of  the 
electric  current. 

"With  a  sufficiently  powerful  electro-motive  force  we 
can  produce  such  a  stress  in  the  medium  in  an  ordinary 
room,  or,  in  other  words,  we  can  polarize  the  medium 
and  make  it  such  a  storehouse  of  electric  energy,  that 
we  can  light  a  little  electric  lamp  anywhere  in  the  room 
without  wires.  Carrying  this  lamp  in  our  hand,  it  will 
light  up  when  we  enter  the  room  and  be  extinguished 
when  we  leave  it.  Tesla  has  shown  the  possibility  of 
this  by  making  the  room  a  great  Leyden  jar,  of  which 
the  walls  form  the  opposite  coatings  and  the  air  takes  the 
place  of  the  glass.  The  charge  upon  the  coatings  of 
such  a  jar,  or,  in  other  words,  the  walls  of  the  room,  is 
made  to  alternate  with  great  rapidity,  and  electric  waves 
fill  the  air,  giving  periodic  to-and-fro  movements  to  it. 


192  WHAT  IS  ELECTRICITY? 

The  molecules  in  a  little  rarefied  bulb  are  thus  set  in 
very  rapid  motion,  and  by  their  impact  on  suitable  sub- 
stances can  raise  the  latter  to  incandescence,  and  by  their 
mutual  collisions  can  also  fill  the  rarefied  tube  with  a 
luminosity. 

Tesla,  in  his  remarkable  lectures  on  the  effects  pro- 
duced by  currents  of  high  frequency,  shows  that  a  suit- 
ably constructed  lamp  can  be  made  to  glow  in  any 
space  by  being  connected  merely  with  one  terminal  of 
a  transformer.  A  little  motor  can  also  be  made  to  re- 
volve by  being  attached  to  one  wipe ;  the  ordinary  elec- 
tric motor  requiring  two  wires,  one  connected  to  the  posi- 
tive pole  of  the  dynamo  and  the  other  to  the  negative  pole. 
In  considering  these  experiments  we  must  remember, 
however,  that  the  electric  circuit  in  both  cases  of  the 
light  and  the  motor  is  completed  through  the  medium 
from  the  positive  pole  of  the  exciter  to  the  negative 
pole.  In  other  words,  lines  of  electrostatic  force  extend 
through  the  medium,  through  the  walls  of  the  room, 
back  to  the  negative  pole  of  the  generator.  We  can 
not  isolate  an  electric  effect  at  one  spot,  or  consider 
that  our  lines  of  stress  stop  at  this  spot.  The  apparatus 
by  means  of  which  Tesla  produced  remarkable  luminous 
effects  is  similar  to  that  of  Thomson,  and  can  be  char- 
acterized as  a  step-up  transformer  of  the  second  order. 
Instead,  however,  of  using  an  alternating  machine  of 
comparatively  low  rate  of  alternation,  Tesla  employed 
one  giving  about  fifty  thousand  alternations  per  second. 
With  the  very  high  potentials  obtained  in  his  second 
step-up  transformer  he  was  able  to  excite  the  molecules 
of  rarefied  gases  to  a  rapid  rate  of  movement,  and  by 
their  impact  on  suitable  matter  to  produce  vivid  light. 

Prof.  Crookes's  experiments  on  radiant  matter  in 
highly  exhausted  tubes  may  be  said  to  have  first  drawn 


STEP-UP  TRANSFORMERS. 


193 


inventors'  minds  to  the  possibility  of  obtaining  light  by 
other  means  than  by  employing  steady  currents  of  elec- 
tricity to  raise  matter  to  incandescence.  In  one  form  of 
the  Crookes  tubes  one  terminal  of  a  Kuhmkorff  coil  ends 
in  a  concave  mirror  (Fig.  28) 
inside  an  exhausted  globe. 
At  the  focus  of  this  globe 
(fy  is  placed  a  bit  of  plati- 
num on  a  glass  stem.  The 
energy  streaming  from  a  is 
reflected  to  the  focus  b,  and 
raises  the  bit  of  platinum  to 
incandescence.  This  lamp 
may  be  said  to  be  the  fore- 
runner of  the  later  attempts 
of  Tesla  to  produce  light  by 
high  electro  -  motive  force. 
The  tube  is  of  importance 
also  in  X-ray  photography. 
The  commercial  employment 
of  strong  to-and-fro  currents 
enables  one  to  greatly  mag- 
nify the  results  obtained  by 
Crookes. 

The  experiments  of  Tesla 
to  obtain  a  light  with  a  small 

amount  of  expenditure  of  energy  are  extremely  sug- 
gestive. At  present,  however,  in  order  to  produce  the 
luminescence  of  such  economical  lamps  we  are  com- 
pelled to  employ  powerful  dynamos  and  transformers. 
We  need  an  engine  of  at  least  ten  horse  power  to  pro- 
duce the  conditions  for  such  an  economical  lamp  of  a 
few  candle  power.  The  problem  is,  to  produce  the  con- 
ditions economically.  I  can  illustrate  this  problem  by 


FIG.  28. 


194  WHAT  IS  ELECTRICITY? 

a  species  of  argum^ntum  ad  hominem.  An  electric 
light  of  feeble  power  can  be  produced  by  shaking  a 
small  amount  of  mercury  in  a  glass  tube  which  has  been 
partially  exhausted  of  air.  The  friction  of  the  mercury 
against  the  glass  walls  of  the  tube  produces  an  electri- 
fication which  in  turn  leads  to  a  heightened  oscillation 
or  movement  of  the  molecules  of  air  still  left  in  the  tube. 
No  heat  can  be  detected  in  this  light.  We  have  pro- 
duced considerable  light  by  electrical  excitation  with 
very  little  heat,  but  the  amount  of  heat  supplied  to  the 
human  body  in  the  shape  of  food  is  very  great,  and  we 
have  to  go  through  an  expensive  transformation  to 
produce  our  little  light. 

It  will  be  well  at  this  stage  of  our  study  of  trans- 
formations of  energy  accomplished  by  the  inventions  of 
man  to  examine  into  the  degree  of  perfection  which  has 
been  obtained.  The  efficiency  of  any  engine  is  the 
amount  of  work  it  can  perform  compared  with  the 
amount  of  energy  given  to  it  in  the  shape  of  fuel.  "  The 
steam  engine  has  an  efficiency  of  about  ten  per  cent ; 
the  efficiency  of  the  best  dynamo  machines  is  ninety 
per  cent ;  therefore  only  nine  per  cent  of  the  energy 
in  the  coal  is  transformed  into  electrical  energy.  In  the 
conversion  of  this  electrical  energy  into  light  about  ten 
per  cent  is  lost  in  the  conductors,  and  we  have  conse- 
quently in  the  lamps  only  0*081  of  the  energy  in  the  coal. 
Of  this  energy  in  the  lamp  about  ninety  per  cent  is  ex- 
pended in  producing  heat,  ten  per  cent  only  being  use- 
ful for  the  production  of  light.  Thus,  the  efficiency  of 
the  electric  lamp  is  only  0-0081,  or  about  one  per  cent."  * 

This  result  is  well  calculated  to  repress  any  feel- 
ing of  exultation  we  may  have  in  contemplating  the 


Palaz,  Industrial  Photometry,  p.  265. 


STEP-UP  TRANSFORMERS.  195 

great  field  which  has  been  opened  by  the  transforma- 
tions of  energy  accomplished  by  the  dynamo  machine. 
We  can  congratulate  ourselves,  however,  that  the  elec- 
tric light  is  more  efficient  than  other  forms  of  light. 
It  has  been  computed  that  the  energy  consumed  hi 
producing  a  light  of  sixteen-candle  power  by  kerosene 
is  42'86  watts  (a  watt  is  -^5-  of  a  horse  power).  An 
Argand  gas  burner  of  twenty-two-candle  power  con- 
sumes 68*8  watts ;  an  incandescent  electric  light,  3*5 
watts  per  candle;  the  arc  light,  0'8  watt  per  candle. 

We  fail,  thus,  in  utilizing  the  energy  in  the  coal, 
and  when  we  produce  a  light  we  convert  most  of  the 
email  amount  of  energy  we  obtain  from  the  coal  into 
nonluminous  heat  waves.  It  has  been  computed  that 
ninety -five  per  cent  of  the  energy  expended  in  produ- 
cing a  light  goes  to  the  production  of  waves  of  the  ether 
which  do  not  affect  the  eye.  Lodge  remarks  that  we 
are  in  the  condition  of  an  organist  who,  in  order  to 
sound  certain  high  notes  of  his  instrument,  is  com- 
pelled to  sound  all  those  of  the  keyboard.  Two  great 
practical  questions  in  the  transformation  of  energy  thus 
confront  us :  How  to  utilize  to  a  greater  degree  the 
energy  stored  up  in  the  coal,  and  how  to  produce  a  light 
rich  only  in  those  rays  which  appeal  to  our  senses  as  light. 

Ebert  *  has  constructed  an  economical  lamp  in  the 
following  manner :  A  is  an  exhausted  glass  globe  (Fig. 
29)  with  an  inner  glass  stem,  B,  on  the  end  of  which  is 
a  paste  of  phosphorescent  paint — "  Griin  blaue  Leucht 
farbe."  E,  and  Ea  are  tin-foil  rings,  which  are  glued  to 
the  glass  globe.  These  rings  are  connected  to  the  two 
terminals  of  the  finer  coil  of  the  step-up  transformer. 
Under  the  action  of  the  to-and-fro  currents  of  the  trans- 


*  Ann.  der  Physik  und  Chemie,  No.  9,  1894. 
14 


196 


WHAT  IS  ELECTRICITY? 


former  the  plates  E,  and  E,  are  electrified,  and  tlie  lines 
of  force  fluctuating  through  the  rarefied  globe  raise  the 
phosphorescent  paint  to  a  high  degree  of  luminosity. 
Ebert  calculates  that  such  a  lamp  consumes  from  1,500 
to  2,000  times  less  energy  than 

Xthe  amylacetate  unit  lamp,  and 
that  this  amount  of  energy  is  in 
the  neighbourhood  of  one  mill- 
ionth of  a  watt,  the  watt  being 
y|-ff  of  a  horse  power.  In  order 
to  avoid  the  inevitable  losses 
which  arise  in  conveying  such 
high-tension  effects  any  distance, 
Ebert  suggests  that  a  little  step- 
up  transformer  can  be  put  in  the 
base  of  each  little  lamp.  The  ne- 
cessity of  electrical  turning  in  the 
experiments  of  Ebert  is  perhaps 
the  most  interesting  fact  in  re- 
gard to  the  endeavour  to  obtain  an  economical  lamp  by 
means  of  to-and-fro  high-tension  currents.  He  found 
that  unless  the  circuit  in  which  the  lamp  was  placed 
was  in  resonance  with  the  exciting  circuit,  the  lamp 
did  not  light  up  to  its  full  brilliancy. 

In  an  interesting  paper  on  the  cheapest  form  of 
light,  Prof.  Langley  calls  attention  to  the  fact  that  there 
is  an  enormous  waste  of  energy  in  the  ordinary  methods 
of  producing  illumination.  In  the  ordinary  Argand- 
burner  gas  flame  this  waste  for  illumination  purposes  can 
be  shown  to  be  something  over  ninety-nine  per  cent  of 
the  radiant  energy  emitted  by  the  lamp.  As  Prof.  Lang- 
ley  points  out,  "  this  waste  comes  from  the  necessity  of 
expending  a  large  amount  of  heat  in  invisible  forms,  and 
each  increase  of  light  represents  not  only  the  small 


FIG.  29. 


STEP-UP  TRANSFORMERS.  197 

amount  of  heat  directly  concerned  in  the  making  of  the 
light  itself,  but  a  new  indirect  expenditure  in  the  pro- 
duction of  invisible  calorific  rays.  Our  eyes  recognise 
heat  mainly  as  it  is  conveyed  in  certain  rapid  ethereal 
vibrations  associated  with  high  temperatures,  while 
we  have  no  usual  way  of  reaching  these  high  tempera- 
tures without  passing  through  the  intermediate  low 
ones,  so  that  if  the  vocal  production  of  a  short  atmos- 
pheric vibration  were  subject  to  analogous  conditions,  a 
high  note  could  never  be  produced  until  we  had  passed 
through  the  whole  gamut,  from  discontinuous  sounds  be- 
low the  lowest  bass,  up  successively  through  every  lower 
note  of  the  scale  till  the  desired  alto  was  attained."  * 

The  phenomena  of  phosphorescence  as  it  is  mani- 
fested in  fireflies,  seems  to  form  an  exception  to  this 
rule.  The  light  emitted  by  this  insect  can  be  produced 
artificially  by  raising  a  body  to  2,000°  F.  No  sensible 
heat,  however,  accompanies  the  firefly's  light,  and  in- 
deed this  can  also  be  said  of  the  light  in  Geissler  tubes. 
It  is  assumed  that  the  firefly's  light  is  produced  with- 
out the  invisible  heat  that  accompanies  our  usual  pro- 
cesses, for  the  spectrum  of  the  firefly's  light  falls  off 
more  rapidly  toward  the  red  end  than  the  spectrum  of 
a  candle,  for  instance.  Prof.  Langley,  in  his  Memoir, 
which  we  have  quoted,  gives  in  detail  the  delicate  meas- 
urements with  his  bolometer,  by  which  he  obtained  the 
following  results  with  the  Cuban  firefly  (Pyrophorus 
noctilucus),  an  insect  about  one  inch  and  a  half  long  and 
half  an  inch  wide.  He  concludes  that  Nature  produces 
this  cheapest  light  with  about  the  four  hundredth  part 
of  the  cost  of  the  energy  which  is  expended  in  the  can- 
dle flame. 

*  American  Journal  of  Science,  vol.  cxl,  1890. 


CHAPTEE  XYL 

LIGHTNING. 

BY  means  of  the  alternating-current  dynamo  and 
by  the  Kuhmkorff  coil  we  have  been  able  to  transform 
the  energy  of  coal  into  the  energy  manifested  by  light- 
ning, for  the  long  sparks  of  Thomson  are  lightning 
discharges.  Electro-magnetic  waves  from  the  sun  pro- 
duced the  coal,  and  we  shall  see  later  that  from  the 
coal,  and  by  means  of  the  dynamo  and  the  step-up 
transformers,  we  can  obtain  again  electro-magnetic 
waves.  After  one  hundred  and  fifty  years  we  have 
come  back  to  the  study  of  sparks  and  the  behaviour  of 
Leyden  jars.  We  have  returned  to  the  domain  of  science, 
which  Benjamin  Franklin  may  be  said  to  have  sud- 
denly illumined  when  he  drew  lightning  from  the 
clouds. 

We  return,  however,  after  a  long  study  of  the 
mechanical  equivalent  of  heat  and  with  scientific  knowl- 
edge greatly  increased  by  exact  measurements.  We 
are  conscious  of  the  great  truth  that  whenever  we  can 
test  our  electrical  theories  by  heat  experiments,  follow- 
ing out  the  line  of  work  indicated  by  Count  Rumford, 
we  are  certain  to  obtain  a  residuum  of  truth.  The  study 
of  the  transformations  of  energy  by  means  of  instruments 
which  measure  the  equivalence  in  heat  of  the  electrical 

198 


LIGHTNING.  199 

actions  we  observe  is  the  final  method  we  must  adopt 
to  test  any  physical  theory  of  electricity. 

Before  the  epoch  of  Count  Kumford  the  amount  of 
exact  experimentation  in  the  subject  of  physics  was  ex- 
tremely small,  and  it  is  not  to  be  wondered  at  that  men 
attributed  to  mysterious  effluvia,  to  caloric,  and  to 
phlogiston  the  cause  of  the  various  transformations  of 
energy  which  they  witnessed.  They  had  no  measures 
of  comparison.  Benjamin  Franklin's  experiments  have 
stood  the  test  of  time,  but  his  theory  of  electricity  has 
long  since  ceased  to  have  value  in  the  scientific  world, 
largely  because  it  had  not  the  weight  of  quantitative 
measurements  behind  it.  Let  us  look  at  his  theory  for 
a  moment,  and  then  examine  our  present  conceptions  of 
the  lightning  flash,  which  was  of  such  absorbing  inter- 
est to  him. 

The  following  account  of  Franklin's  fluid  theory  of 
electricity  was  presented  to  the  Koyal  Society  in  1851 
by  William  "Watson,  F.  E.  S. : 

"This  ingenious  author  (Franklin),  from  a  great 
variety  of  curious  and  well -adapted  experiments,  is  of 
opinion  that  the  electrical  matter  consists  of  particles 
extremely  subtile,  since  it  can  permeate  common  matter, 
even  the  densest  metals,  with  such  ease  and  freedom  as 
not  to  receive  any  perceptible  resistance;  and  that  if 
any  one  should  doubt  whether  the  electrical  matter 
passes  through  the  substance  of  bodies  or  only  over 
and  along  their  surfaces,  a  shock  from  an  electrified 
large  glass  jar  taken  through  his  own  body  will  proba- 
bly convince  him. 

"  Electrical  matter,  according  to  our  author,  differs 
from  common  matter  in  this,  that  the  parts  of  the 
latter  mutually  attract,  and  those  of  the  former  mutu- 
ally repel  each  other,  hence  the  divergency  in  a  stream 


200  WHAT  IS  ELECTRICITY? 

of  electrified  effluvia ;  but  that  though  the  particles  of 
electrical  matter  do  repel  each  other,  they  are  strongly 
attracted  by  all  other  matter.  From  these  three  things, 
viz.,  the  extreme  subtilty  of  the  electrical  matter, 
the  mutual  repulsion  of  its  parts,  and  the  strong  at- 
traction between  them  and  other  matter,  arises  this 
effect,  that  when  a  quantity  of  electrical  matter  is  ap- 
plied to  a  mass  of  common  matter  of  any  bigness  or 
length  within  our  observation  (which  has  not  already 
got  its  quantity),  it  is  immediately  and  equally  diffused 
through  the  whole. 

"  Thus  common  matter  is  a  kind  of  sponge  to  the 
electrical  fluid ;  and  as  a  sponge  would  receive  no  water 
if  the  parts  of  water  were  not  smaller  than  the  pores 
of  the  sponge,  and  even  then  but  slowly  if  there  was 
not  a  mutual  attraction  between  those  parts  and  the 
parts  of  the  sponge,  and  would  still  imbibe  it  faster  if 
the  mutual  attraction  among  the  parts  of  the  water  did 
not  impede,  some  force  being  required  to  separate 
them,  and  faster  if,  instead  of  attraction,  there  were  a 
mutual  repulsion  among  those  parts  which  would  act  in 
conjunction  with  the  attraction  of  the  sponge,  so  is  the 
case  between  the  electrical  and  common  matter.  In 
common  matter,  indeed,  there  is  generally  as  much  of 
the  electrical  as  it  will  contain  within  its  substance ;  if 
more  is  added,  it  lies  without  upon  the  surface  and 
forms  what  we  call  an  electrical  atmosphere,  and  then 
the  body  is  said  to  be  electrified." 

"  Common  fire  is  in  all  bodies,  more  or  less,  as  well 
as  electrical  fire.  Perhaps  they  may  be  different  modi- 
fications of  the  same  element,  or  they  may  be  different 
elements.  The  latter  is  by  some  suspected.  If  they 
are  different  things,  yet  they  may  and  do  subsist  to- 
gether in  the  same  body.  When  electrical  fire  strikes 


LIGHTNING.  201 

through  a  body,  it  acts  upon  the  common  fire  in  it,  and 
puts  that  fire  in  motion,  and  if  there  be  a  sufficient 
quantity  of  each  kind  of  fire,  the  body  will  be  in- 
flamed. "When  the  quantity  of  common  fire  in  the 
body  is  small,  the  quantity  of  the  electrical  fire  (or  the 
electrical  stroke)  should  be  greater ;  if  the  quantity  of 
common  fire  be  great,  less  electrical  fire  suffices  to  pro- 
duce the  effect.  .  .  .  Metals  are  often  melted  by  light- 
ning, though  perhaps  not  from  heat  in  the  lightning, 
nor  altogether  from  agitated  fire  in  the  metals.  For, 
as  whatever  body  can  insinuate  itself  between  the  par- 
ticles of  metals  and  overcome  the  attraction  by  which 
they  cohere  (as  sundry  menstrua  can)  will  make  the 
solid  become  a  fluid,  as  well  as  fire,  yet  without  heating 
it,  so  the  electrical  fire,  or  lightning,  creating  a  violent 
repulsion  between  the  particles  of  the  metal  it  passes 
through,  the  metal  is  fused."  * 

Franklin  had  no  measuring  instruments  or  insulated 
wire  at  hand,  and  his  entire  electrical  apparatus  con- 
sisted practically  of  merely  an  electrical  machine  and 
Leyden  jars.  To  him  the  great  manifestation  of  atmos- 
pheric electricity  in  the  lightning  flash  constituted  the 
chief  object  of  study.  He  bestowed  no  thought  on 
what  is  now  the  great  object  of  our  scientific  study — 
the  dielectric  which  is  pierced  and  shattered  by  the 
lightning  discharge. 

Let  us  return  for  a  moment  to  the  object  of  Frank- 
lin's studies,  the  lightning  discharge,  and  examine  it  by 
our  modern  methods. 

It  is  well  known  that  when  air  is  subjected  to  a 
sudden  strain  at  the  moment  of  an  electrical  discharge 
it  acts  like  glass  or  a  similar  elastic  solid,  and  is  cracked 

*  Works  of  Benjamin  Franklin.    Jared  Sparks.    Vol.  v.,  p.  221. 


202  WHAT  IS  ELECTRICITY! 

in  zigzag  fissures ;  indeed,  the  resemblance  between  the 
ramifications  of  lightning  and  the  seams  produced  in 
plates  of  glass  by  pressure  has  been  commented  upon 
by  various  observers.  Photographs  of  powerful  elec- 
tric sparks  lead  one  to  conclude  that  a  discharge  of 
lightning  makes  way  for  its  oscillations  by  first  break- 
ing down  the  resistance  of  the  air  by  means  of  a  dis- 
ruptive pilot  spark ;  through  the  hole  thus  made  in  the 
air  the  subsequent  surgings  or  oscillations  take  place. 

I  have  remarked  in  a  previous  chapter  that  if  a 
powerful  discharge  from  a  Leyden  jar  perforates  a 
piece  of  cardboard  a  burr  is  raised  on  both  sides  of 
the  paper.  The  believers  in  the  two  fluid  theories  of 
electricity  explained  this  phenomenon  by  saying  that 
the  positive  electricity  passed  in  one  direction,  and  the 
negative  in  the  opposite  direction. 

In  order  to  study  more  carefully  the  cause  of  this 
burr  on  both  sides  of  a  sheet  of  cardboard  which  is  per- 
forated by  an  electric  spark,  I  arranged  two  Leyden-jar 
circuits,  one  of  which  was  oscillatory  and  the  other  was 
nonoscillatory ;  and  I  studied  the  perforations  in  a  sheet 
of  cardboard  produced  by  the  two  kinds  of  discharges,  in 
order  to  see  if  the  cause  of  the  burr  on  both  sides  of 
the  paper  was  due  to  the  oscillations  of  the  discharge. 
If  this  were  so,  the  unidirectional  discharge  should  give 
a  burr  only  on  one  side  of  the  paper.  In  order  to 
charge  the  jars  I  employed  an  alternating  dynamo  with 
a  step-up  transformer,  or,  in  other  words,  a  Kuhmkorff 
coil.  The  Leyden  jars  were  connected  to  the  terminals 
of  the  Ruhmkorff,  and  in  this  manner  a  torrent  of  sparks 
was  produced.  Photographs  of  the  sparks  from  the  jars 
were  obtained  by  means  of  a  revolving  mirror.  Having 
got  a  suitable  oscillatory  discharge,  a  disk  of  card- 
board was  mounted  on  a  revolving  shaft,  and  was  per- 


LIGHTNING. 


203 


forated  by  the  sparks  when  in  rapid  movement.  In  this 
way  a  large  number  of  perforations  could  be  studied. 
A  liquid  resistance  was  then  inserted  in  the  circuit  of 
the  Leyden  jars,  and  increased  until  the  photograph 
taken  showed  that  the  oscillations  had  been  damped  by 
the  resistance,  and  only  a  unidirectional  discharge  re- 
mained. The  revolving  disk  of  cardboard  was  then 
perforated  by  these  unidirectional  sparks,  and  on  com- 
paring the  two  sets  of  perforations  it  was  found  that 
the  burr  occurred  on  both  sides  of  the  paper  as  well 
with  the  unidirectional  sparks  as  with  the  oscillatory 
sparks.  It  therefore  does  not  arise  from  the  to-and-fro 
movement  of  electricity.  I  am  inclined  to  consider  it 
as  due  to  an  explosion  in  the  paper  produced  by  the 
heated  air.  It  is  well  known  that  gunpowder  can  not 
be  fired  by  the  spark  from  a  Leyden  jar  unless  one  in- 
terposes a  wet  string  in  the  circuit.  This  wet  string 
acts  like  the  liquid  resistance  in  the  above  experiments ; 
it  damps  the  oscillation,  and  diminishes  the  explosive 
effect  of  the  heated  air,  which  drives  the  particles  of 
gunpowder  asunder.  The  phenomenon  certainly  has 
no  bearing  upon  the  two  fluid  theory  of  electricity. 

In  examining  the  early  photographs,  taken  by  Fed- 
dersen,  of  electric  sparks,  one  perceives  that  the  elec- 
tric oscillations  tend  to  follow,  for  at  least  some  hun- 
dred thousandths  of  a  second,  the  path  made  by  the 
pilot  spark ;  and  there  are  observers  who  believe 
that  by  rapidly  moving  a  camera  they  have  obtained 
evidence  that  successive  discharges  of  lightning  follow 
the  same  path.  Prof.  Lodge  has  protested,  with  rea- 
son, against  the  conclusions  drawn  by  the  method  of 
"waggling"  the  head  or  a  camera;  for  the  movement 
of  the  head  or  the  camera  certainly  requires  the  hun- 
dredth of  a  second,  while  the  discharge  of  lightning 


204  WHAT  IS  ELECTRICITY? 

is  over  in  less  than   one  hundred  thousandth   of  a 
second. 

The  method  of  photographing  electrical  discharges 
by  means  of  a  revolving  mirror  seems  to  be  the  best 
method  of  studying  the  behaviour  of  air  which  is  sud- 
denly subjected  to  the  electric  strain.  I  have  there- 
fore examined  this  behaviour  with  more  powerful 
means  than  those  employed  by  previous  observers ;  and 
it  may  be  well  to  recall  here  the  fact  that  in  lightning 
discharges  high  electro -motive  force  and  great  quantity 
are  frequently  combined  in  a  very  short  interval  of 
time.  The  modern  alternating  machine,  therefore,  and 
the  device  of  the  transformer  enable  one  to  study  the 
character  of  lightning  more  successfully  than  is  possible 
by  means  of  an  electrical  machine ;  for  both  the  electro- 
motive force  of  a  discharge  and  its  quantity  can  be  ad- 
justed over  a  wide  range.  In  my  study  of  this  subject  I 
employed  an  alternating  machine  which  gave  three  hun- 
dred to  four  hundred  alternations  per  second,  and  a  cur- 
rent of  from  fifteen  to  twenty  amperes.  By  means  of  a 
step-up  transformer  and  an  oil  condenser,  discharges  of 
high  electro -motive  force  and  great  quantity  could  be 
readily  obtained.  The  method  of  the  excitation  of  a 
Kuhmkorff  coil  or  transformer  by  means  of  an  alter- 
nating dynamo — due  originally  to  Spottiswoode — has 
placed  in  the  hands  of  the  experimenter,  as  I  have  said, 
powerful  means  of  studying  electric  discharges ;  and  by 
the  device  of  an  air  blast,  or  other  contrivance  for  ob- 
taining a  quick  break  in  the  continuity  of  the  electrical 
discharges,  high  electro-motive  force  can  be  obtained. 
In  certain  sparks  which  I  studied,  the  interval  between 
the  oscillations  was  found  to  be  about  one  hundred  thou- 
sandth of  a  second,  and  the  electrical  discharge  followed 
exactly  the  same  path  in  the  air  for  three  hundred 


LIGHTNING.  205 

thousandths  of  a  second.  During  this  length  of  time 
every  sinuosity  in  the  pilot  spark  is  exactly  reproduced. 
I  employed  terminals  of  tin ;  and,  in  one  case,  a  mass  of 
melted  and  vapourized  tin  remained  suspended  in  the  air 
for  at  least  three  hundred  thousandths  of  a  second  before 
it  was  dissipated  in  a  cometlike  tail.  During  the  three 
hundred  thousandths  of  a  second,  therefore,  the  air  re- 
mained passive  while  the  electrical  oscillations  took  place. 
During  this  time  it  is  fair  to  conclude  that  the  heat  pro- 
duced by  the  passage  of  the  spark  was  not  sensibly  con- 
ducted away.  If  conduction  of  heat  had  taken  place, 
the  electrical  resistance  of  the  air  path  would  have  been 
sensibly  altered,  and  the  path  of  the  discharge  would 
have  changed  in  form.  Here,  I  think,  we  have  an  in- 
teresting limit  to  the  tune  it  takes  atmospheric  air  to 
respond  to  the  phenomenon  of  heat  conduction. 

I  have  said  that  the  discharges  I  employed  were 
powerful  both  in  regard  to  electro-motive  force  and  to 
quantity.  Iron  terminals  one  quarter  of  an  inch  in 
diameter  were  raised  to  a  white  heat  by  the  continuous 
passage  of  the  sparks,  and  globules  of  the  melted  metal 
were  formed.  When  the  sparks  were  passed  through 
the  secondary  of  a  transformer  three  fifty-volt  Edison 
lamps  placed  in  multiple  in  the  primary  of  the  trans- 
former, which  consisted  of  merely  two  layers  of  thick 
copper  wire,  were  lighted  to  full  incandescence.  The 
spark  from  two  large  glass  condensers  of  5,000  electro- 
static units  each,  excited  by  an  electrical  machine,  and 
passed  through  the  secondary  of  the  same  step-down 
transformer,  barely  raised  a  six- volt  lamp  in  the  primary 
to  a  red  heat.  The  study  of  the  efficiency  of  step-down 
transformers  in  thus  transforming  transient  currents  of 
high  potential  to  transient  currents  of  low  potential  and 
comparatively  large  current,  enables  one  to  obtain  an 


206  WHAT  IS  ELECTRICITY? 

estimate  of  the  high  potential  of  lightning,  and  of  the 
current  which  accompanies  its  fall  of  potential.  Thus, 
if  C  denote  the  current  in  the  lightning  discharge,  and 
E  the  electro-motive  force,  C'  and  E'  the  corresponding 
quantities  in  the  circuit  of  the  step-down  transformer, 
A  the  efficiency  of  the  transformer,  we  shall  have 

C'E'  =  ACE. 

The  element  of  time  and  the  mode  of  transforma- 
tion must  be  considered  in  any  estimate  of  the  amount 
of  energy  in  a  lightning  discharge.  Although  a  power- 
ful spark  of  electricity  from  two  Leyden  jars,  each  of 
5,000  electrostatic  units,  is  incapable  of  decomposing 
water  directly,  yet  by  its  passage  through  the  secondary 
of  a  step-down  transformer  it  can  decompose  the  water 
with  great  evolution  of  the  gases ;  and  it  is  probable  that 
an  ordinary  discharge  of  lightning  of  a  few  hundred  feet 
in  length,  I  have  before  remarked,  could  light  for  an 
instant  many  thousand  incandescent  lamps  if  it  were 
properly  transformed  by  means  of  a  step-down  trans- 
former. Indeed,  the  ringing  of  electrical  bells  and  the 
melting  of  electrical  fuses  are  of  common  occurrence  dur- 
ing thunder-storms,  and  manifest  the  energy  of  lightning 
discharges.  During  a  recent  visit  at  a  summer  hotel 
which  was  lighted  by  incandescent  lamps,  I  was  much 
interested  to  observe  that  the  lamps  blinked  at  every  dis- 
charge of  lightning,  although  the  interval  which  elapsed 
between  the  blinking  and  the  peals  of  thunder  showed 
that  the  storm  was  somewhat  remote.  This  effect  was 
doubtless  due  to  induction  produced  by  the  surgings  of 
the  lightning  discharges ;  for  in  heavy  and  in  near  dis- 
charges the  lights  were  completely  extinguished,  although 
no  fuses  were  burned.  Electric-light  wires  and  gas  pipes 
should  never  be  contiguous,  for  no  lightning  guard  or 


LIGHTNING.  207 

protector  can  insure  that  minute  sparks,  due  in  some 
cases  to  resonance  effects,  may  not  arise. 

The  study  of  the  disruptive  or  oscillatory  discharge 
of  lightning  is  closely  related  to  that  of  the  brush 
discharge  and  the  phenomenon  of  the  aurora  borealis, 
for  the  disruptive  discharge  ceases  to  be  disruptive  after 
a  few  hundred  thousandths  of  a  second,  and  partakes 
of  the  nature  of  a  brush  discharge.  The  zigzag  fissure 
in  the  air  disappears,  and  only  the  spark  terminals 
glow.  Eecent  experimenters  have  exhibited  as  a  mar- 
vel the  lighting  of  a  vacuum  tube  through  the  human 
body  by  grasping  one  terminal  of  a  suitable  transformer 
with  one  hand  and  by  holding  the  vacuum  tube  in  the 
other  hand.  It  must  be  remembered,  however,  that  the 
lines  of  force  proceed  from  the  hand  which  holds  the 
vacuum  tube  through  the  air  and  the  walls  or  floor  of 
the  room  to  the  other  terminal  of  the  transformer.  We 
can  change  this  brush  discharge  or  luminosity  at  either 
terminal  of  a  transformer  into  a  disruptive  discharge  by 
lessening  the  distance  between  the  terminals  or  by  in- 
creasing the  electro-motive  force. 

I  am  fully  aware  that  the  oscillatory  discharge  of 
lightning  with  its  disruptive  effects,  which  I  have 
noted,  its  permanence  of  path,  and  the  fading  of  the 
disruptive  discharge  into  the  brush  discharge  or  mere 
luminosity  at  either  of  the  spark  terminals,  is  a  far 
simpler  phenomenon  than  the  luminosity  produced  in 
rarefied  tubes ;  for  in  the  latter  phenomenon  we  have 
the  dissociation  and  impact  of  molecules,  and  we  must 
consider  all  the  problems  of  atomic  motion  in  addition 
to  those  of  the  oscillatory  nature  of  electrical  waves.  It 
is  not  my  purpose  to  enter  into  a  consideration  of  the 
molecular  movements  involved  in  oscillatory  discharges 
in  vacuum  tubes;  but  having  discussed  some  of  the 


208  WHAT  IS  ELECTRICITY? 

general  features  of  discharges  of  electricity  in  air  at 
the  ordinary  pressure,  I  shall  endeavour  to  trace  the 
connection  between  such  discharges  and  the  phenome- 
non of  the  aurora  borealis.  To  my  mind,  the  lumi- 
nosity in  a  vacuum  tube,  the  glass  exterior  of  which  is 
held  in  one  hand  while  the  other  hand  grasps  the  ter- 
minal of  a  Ruhmkorff  coil,  closely  represents  the  phe- 
nomenon of  the  northern  lights ;  for  we  have  in  this 
case  a  discharge  of  electricity  from  a  higher  level  to  a 
lower  through  a  rarefied  medium. 

I  have  said  that  we  can  pass  by  insensible  gradations 
from  the  condition  of  the  brush  discharge  to  that  of  the 
disruptive  discharge.  By  intercalating  a  noninductive 
water  resistance  and  a  vacuum  tube  between  the  ter- 
minals of  a  suitable  transformer,  we  can  exactly  imitate 
the  phenomena  observed  when  the  vacuum  tube  is  held 
in  one  hand  while  the  other  hand  grasps  one  terminal 
of  the  transformer.  In  this  case  the  water  resistance 
takes  the  place  of  the  resistance  of  the  air  of  the  room. 
The  intensity  of  the  discharge  being  thus  much  dimin- 
ished, one  can  readily  study  various  manifestations  of 
stratification  which  may,  perhaps,  be  termed  transitory 
stratifications  in  distinction  to  the  stationary  wavelike 
forms  observed  in  narrow  tubes.  The  transitory  strati- 
fications can  be  produced  at  will  by  touching  suitable 
points  of  a  vacuum  tube  with  the  finger  or  by  connect- 
ing such  points  with  the  ground.  Such  stratifications 
are  stationary  as  long  as  the  ground  connection  is  main- 
tained, and  are  independent  of  the  rate  of  the  alternat- 
ing machine  which  excites  the  transformer.  It  is  evi- 
dent that  the  condenser  action  of  the  vacuum  tube  plays 
an  important  part  in  this  phenomenon.  In  observing 
the  striae  and  columnar  form  of  the  waving  of  the  light 
excited  in  this  manner  in  vessels  or  tubes  filled  with 


LIGHTNING.  209 

rarefied  gases,  one  is  led  to  believe  that  the  stratified 
form  of  the  aurora  borealis  is  produced  in  a  similar 
manner. 

The  pulsation,  therefore,  of  the  aurora  is  in  no  way, 
I  believe,  connected  with  any  phenomenon  of  the  oscil- 
latory discharge;  yet  certain  writers  have  intimated 
that  the  glowing  of  vacuum  tubes  which  are  connected 
with  only  one  terminal  of  a  transformer  and  the  light  of 
the  aurora  are  due  to  millions  of  electrical  oscillations 
per  second.  Now  it  is  impossible  to  study  the  question 
of  the  rate  of  oscillation  of  the  brush  discharge  by  means 
of  Fedderson's  method,  for  the  light  of  the  discharge  is 
not  sufficient  to  produce  a  photograph.  A  brief  con- 
sideration, however,  of  the  laws  of  electrical  oscillations 
shows,  I  think,  that  such  writers  are  mistaken  ;  for  the 
rate  of  decay  of  the  amplitude  of  such  oscillations  is  ex- 

Rt 

pressed  by  the  factor  e^.  In  the  case  of  the  brush 
discharge,  although  we  may  be  dealing  with  very  small 
values  of  self-induction,  L,  and  small  values  of  time,  t, 
we  have,  on  the  other  hand,  great  values  of  K.  I  be- 
lieve, therefore,  that  the  brush  discharge  is  reduced  to 
the  case  of  one  throb,  which  is  analogous  to  the  pilot 
spark  in  disruptive  discharges  (Plate  I,  frontispiece).  In 
regard  to  the  aurora,  it  may  be  urged  that  the  resistance 
of  the  rarefied  air  is  not  enormous.  In  answer  to  this  it 
can  be  said  that  the  phenomenon  of  the  aurora  can  be 
best  reproduced  by  intercalating  a  tube  of  rarefied  air 
with  a  very  large  water  resistance  between  the  terminals 
of  a  suitable  transformer.  The  supposition  that  the 
aurora  is  produced  by  the  action  of  extremely  rapid 
electrical  oscillation  on  molecules  of  rarefied  air  is  not 
borne  out  by  the  theory  of  transient  currents ;  and  ex- 
periment shows  that  the  phenomenon  of  the  waving  and 


210  WHAT  IS  ELECTRICITY? 

apparent  stratification  observed  at  times  in  the  aurora 
is  due  to  the  redistribution  of  the  lines  of  force  which 
is  produced  by  suitable  earths  or  conductors  in  the 
shape  of  regions  of  cloud  or  moisture. 

The  comparatively  small  resistance  of  the  electric 
spark  in  air,  noticed  by  many  observers,  is  due,  I  be- 
lieve, to  the  permanence  of  path ;  for  this  path  is  in- 
tensely heated,  and  is  practically  a  charred  hole  in  the 
air.  When  this  path  no  longer  becomes  such  a  hole 
and  the  heated  air  rises  and  is  dissipated,  the  oscilla- 
tions of  the  electric  spark  become  rapidly  damped,  and 
we  have  the  phenomenon  of  the  brush  discharge — a 
glow  at  each  of  the  spark  terminals  without  a  disruptive 
discharge ;  the  lines  of  force  crowding  from  one  ter- 
minal seek  the  other  terminal  through  the  air  of  the  room, 
and  in  passing  through  rarefied  air  the  energy  along  the 
lines  of  force  is  manifested  by  molecular  actions  which 
are  apparently  protean  in  form.  I  see,  therefore,  no 
evidence  for  believing  in  the  rapid  oscillation  of  the 
aurora. 

The  more  or  less  general  use  of  Edison  lamps  en- 
ables one  to  try  many  experiments  which  are  possible 
with  vessels  from  which  the  air  has  been  partially  with- 
drawn. An  incandescent  lamp  suspended  by  its  brass 
socket  from  a  conductor  of  an  ordinary  electrical  ma- 
chine glows  when  the  machine  is  excited ;  the  glow 
does  not  come  from  the  carbon  filament,  but  from  the 
motion  of  the  molecules  of  air  left  in  the  tube,  which 
are  greatly  excited  by  the  electrical  energy  which  is  be- 
ing stored  up  in  this  little  Leyden  jar.  The  whole  in- 
terior of  the  globe  is  filled  with  a  luminescence  which 
seems  to  be  more  intense  along  the  carbon  filament. 

If  one  takes  one  of  these  globes,  the  filament  of 
which  is  broken  and  which  still  preserves  its  partial 


LIGHTNING.  211 

vacmim,  one  can  store  up  in  it  sufficient  energy  to 
provide  luminescent  effects  for  it  may  be  an  hour 
after  the  lamp  is  removed  from  the  machine.  To  ac- 
complish this,  one  should  hold  the  bulb  of  the  lamp  in 
the  hand  and  charge  the  broken  filament  through  the 
brass  socket  by  touching  the  latter  to  the  conductor 
of  the  electrical  machine.  When  the  bulb  is  removed 
from  the  machine  the  light  in  it  disappears.  It  can 
be  made  to  reappear,  however,  at  will  by  simply  touch- 
ing the  socket  to  the  table,  the  floor,  or  the  wall  of 
the  room.  One  can  thus  provide  himself  with  a  little 
electrical  lamp,  which  he  can  light  by  simply  touching 
it  to  the  wall  of  the  room,  or  to  another  person's  body, 
or  to  a  piece  of  tin  foil  glued  to  a  plane  of  glass.  Its 
light  is  feeble,  to  be  sure.  It  is  like  a  firefly's  light — 
a  mysterious  radiance.  In  watching  its  fluctuations  as 
one's  hand  is  moved  over  the  glass  globe  one  is  forcibly 
reminded  of  the  streaming  of  the  aurora  borealis,  and 
one  can  not  but  conclude  that  the  wavering  light  of 
this  phenomenon  is  due  to  the  same  cause — the  slow 
discharge  through  rarefied  air  of  the  electrical  charge 
on  the  condenser  formed  by  the  upper  layers  of  clouds 
and  the  lower  strata  of  humid  air.  Not  all  incandes- 
cent lamps  enable  one  to  thus  store  up  luminescence. 
If  the  lamp,  however,  preserves  a  suitable  degree  of 
rarefaction  the  phenomenon  I  have  described  can  readily 
be  produced. 

"With  a  suitably  broken  carbon  filament  an  interest- 
ing electrostatic  effect  can  be  noticed  also  with  Edison 
glow  lamps.  Holding  the  bulb  in  one  hand  and  bring- 
ing the  brass  socket  in  contact  with  the  positive  prime 
conductor  of  an  electrical  machine,  the  little  globular 
condenser  receives  a  charge.  On  touching  the  table 
with  the  brass  socket  the  filament  immediately  begins 
15 


212  WHAT  IS  ELECTRICITY? 

to  vibrate,  and  ceases  to  vibrate  when  it  is  held  in 
the  air,  not  touching  any  object.  Immediately  on 
touching  the  table,  the  floor,  the  wall,  or  any  object, 
the  filament  begins  to  vibrate  again,  -causing  a  ringing 
note  like  that  of  a  feeble  electric  bell.  Indeed,  it  is 
an  electric  bell,  which  can  be  made  to  ring  for,  it  may 
be,  an  hour  after  the  charging.  It  is  evident  that  the 
broken  filament  either  positively  or  negatively  charged 
is  attracted  to  the  oppositely  charged  glass  surface, 
and,  having  lost  a  portion  of  its  charge,  its  elasticity 
causes  it  to  swing  back,  and  by  its  connection  with 
the  ground  through  the  table  a  difference  of  potential 
is  again  established  between  it  and  the  charged  glass 
surface,  and  the  phenomenon  is  repeated.  If  a  lamp 
with  unbroken  filament  is  charged  by  thus  holding  it 
in  contact  with  the  conductor  of  the  electrical  machine 
and  is  afterward  lighted  by  an  electric  current  the 
glowing  filament,  if  it  is  a  thin  and  flexible  one,  wiU 
continue  to  vibrate  for  many  minutes. 

The  cause  of  atmospheric  electricity  is  not  well 
understood.  It  is  certain,  however,  that  it  is  not  due 
to  the  evaporation  of  water,  for  exhaustive  experiments 
have  never  detected  the  slightest  electrification  due  to 
evaporation.  The  friction  of  particles  of  water,  how- 
ever, against  material  substances  is  abundantly  able  to 
produce  a  high  electrification.  This  is  proved  by  the 
Armstrong  electrical  machine,  by  means  of  which  jets 
of  water  spray  were  forced  through  nozzles,  which  be- 
came strongly  electrified.  A  curious  case  of  this  action 
lately  came  to  my  attention.  The  operators  on  a  tele- 
phone circuit  were  much  troubled  by  sparks  occurring 
on  the  line,  and  it  was  found  that  the  circuit  was  elec- 
trified by  means  of  a  locomotive  which,  stationed  on  a 
side  switch,  blew  off  steam  against  the  overhead  wires. 


LIGHTNING.  213 

The  friction  of  dust  particles  must  also  be  a  potent 
cause  of  electrification.  The  tops  of  the  pyramids  in 
sand  storms  are  strongly  charged.  Prof.  Lodge  has 
made  the  following  suggestive  remark : *  "It  has  been 
discovered  by  meteorologists  that  thunderstorms  are 
often  associated  with  curious  Y-shaped  troughs  or  de- 
pressions among  the  isobars,  evidencing  a  whirl  or 
cyclone  with  its  axis  horizontal.  Now  I  would  suggest 
that  a  horizontal  cyclone  is  very  like  a  cylinder  electrical 
machine,  with  the  earth  acting  as  rubber  and  the  upper 
regions  of  air  acting  as  prime  conductor,  the  air  which 
has  been  charged  by  friction  being  discharged  as  soon 
as  it  is  carried  up  to  these  higher  regions  and  thus  elec- 
trifying them  continually  until  they  locally  discharge." 

The  effect  of  the  electrification  of  the  particles  of 
water  vapour  is  to  cause  them  to  unite  into  large  drops. 
This  phenomenon  is  often  noticed  in  thundershowers  in 
a  heavy  fall  of  large  drops.  An  experiment  due  to 
Lord  Eayleigh  illustrates  this  in  a  beautiful  manner.  A 
little  vertical  fountain  is  produced  by  connecting  a 
piece  of  rubber  tubing  provided  with  a  glass  nozzle  to 
an  ordinary  water  pipe  or  other  suitable  supply.  When 
a  piece  of  electrified  sealing  wax  is  brought  near  the 
fountain  the  finer  drops  coalesce  into  larger  ones  and 
the  jet  changes  in  form.  Electrification  of  fine  water 
vapour  can  thus  cause  clouds  to  deposit  their  vapour  in 
the  form  of  rain  by  bringing  the  fine  particles  together 
and  by  thus  increasing  the  weight  of  the  drops. 

The  northern  light  I  have  said  can  be  supposed  to 
be  brushlike  discharges  in  the  higher  rarefied  regions  to 
lower  regions.  The  brush  discharge  is  plainly  seen 
about  an  ordinary  electrical  machine  when  we  approach 

*  Lightning  Conductors  and  Lightning  Guards,  Lodge,  p.  3. 


214:  WHAT  IS  ELECTRICITY? 

a  finger  to  the  conductors  in  a  dark  room.  TThen  we 
bring  the  finger  near  enough  to  the  conductor  we  ob- 
tain a  spark,  and  it  is  this  phenomenon,  and  not  the 
brush  discharges,  that  we  perceive  in  lower  latitudes  in 
the  case  of  an  ordinary  thunderstorm. 

The  work  done  in  these  so-called  silent  discharges, 
like  the  aurora  or  in  the  ordinary  brush  discharge,  is 
small  compared  with  that  done  by  the  lightning  dis- 
charge, as  we  can  see  roughly  by  the  differences  in  the 
intensity  of  the  light  produced.  I  have  tested  this 
question  of  difference  of  work  in  the  following  man- 
ner :  A  Leyden  jar,  with  its  outer  coating  slit  so  as 
to  produce  alternate  spaces  of  tin  foil  and  glass,  was 
charged  to  a  sufficient  degree  to  produce  a  spark  be- 
tween terminals  a  fixed  distance  apart.  The  spark  was 
examined  by  a  revolving  mirror,  and  the  number  of 
oscillations  or  surgings  to  and  fro  was  noted.  At 
each  discharge  between  the  spark  terminals  a  brush 
discharge  occurred  between  the  slits  in  the  coating  of 
the  jar.  When  the  jar  was  placed  in  oil  this  brush 
discharge  ceased,  but  no  essential  diminution  could  be 
perceived  in  the  energy  manifested  in  the  spark.  The 
number  of  oscillations  were  the  same,  and  the  duration 
of  the  spark  was  not  apparently  modified. 


CHAPTER  XYII. 

WAVE   MOTION. 

OUE  study  of  electricity  leads  us  now  to  the  general 
subject  of  wave  motion,  which  up  to  the  time  of  the 
laying  of  the  Atlantic  cable  seemed  to  be  very  little  in 
touch  with  practical  life.  It  was  a  subject  for  mathe- 
maticians and  the  natural  philosophers,  and  it  seemed  to 
have  no  commercial  importance.  In  signalling,  how- 
ever, through  the  cable  the  practical  man  was  speedily 
confronted  with  problems  of  wave  motion,  and  with 
the  invention  of  the  telephone  the  study  of  wave  mo- 
tion became  instantly  of  importance  to  the  practical 
electrician.  The  progress  of  electricity  is  steadily  in 
the  direction  of  the  economical  production  of  wave 
motion. 

"  By  a  wave  is  understood  a  state  of  disturbances 
which  is  propagated  from  one  part  of  a  medium  to 
another."  Energy  pauses,  and  not  matter.  "Waves  are 
free  or  forced.  An  example  of  a  free  wave  is  afforded 
by  that  of  the  wave  running  into  the  Bay  of  Fundy, 
which  is  almost  free  from  the  influence  of  the  sun  or 
moon ;  while  the  ocean  tide  is  a  forced  wave,  since  it 
depends  upon  the  continued  action  of  the  moon  and 
sun. 

It  has  been  computed  that  waves  on  the  ocean  of 
about  three  hundred  feet  long  travel  at  the  rate  of 


216 


WHAT  IS  ELECTRICITY  t 


nearly  forty  feet  per  second,  or  twenty-seven  miles  per 
hour.  Their  disturbance,  however,  is  merely  superficial. 
Even  if  they  are  forty  feet  high,  the  disturbance  of  a 
water  particle  at  a  depth  of  three  hundred  feet  is  not 
quite  half  an  inch  from  its  mean  position.  The  depths 
of  the  ocean  are  practically  undisturbed  by  such  waves 
on  the  surface  (Prof.  Tait). 

Although  the  study  of  wave  motions  of  heavy 
fluids,  like  water,  or  even  air,  may  provide  us  with 
analogies  by  means  of  which  we  can  illustrate  wave  mo- 
tions in  an  attenuated  medium  like  the  ether,  we  must 
bear  constantly  in  mind  the  fact  that  the  viscosity  of 
water  or  that  of  the  air  greatly  modifies  the  circum- 
stances of  wave  motion. 

Our  ideas,  however,  of  waves  in  the  ether  of  space, 
which  are  believed  to  convey  the  energy  of  the  sun  to 
us,  are  primarily  obtained  from  con- 
templation of  the  wave  motions 
which  we  perceive  in  water  and  the 
air.  The  electric  spark  has  been 
used  in  an  interesting  manner  to 
make  manifest  waves  in  air  which 
otherwise  would  escape  our  senses. 
Prof.  Boys*  by  its  aid  has  photo- 
graphed the  waves  caused  by  the 
motion  of  a  bullet.  His  method  is 
substantially  as  follows : 

C  is  a  plate  of  window  glass  (Fig. 
30)  with  a  square  foot  of  tin  foil  on 
both  sides.  This  constitutes  the  con- 
denser, and  it  is  charged  until  its  potential  is  not  suffi- 
cient to  make  a  spark  at  each  of  the  gaps,  E  and  E', 


FIG.  30. 


*  Nature,  March  9,  1893. 


WAVE  MOTION.  217 

though  it  would,  if  either  one  of  these  were  made  to 
conduct,  immediately  cause  a  spark  at  the  other ;  c  is 
a  Leyden  jar  of  very  small  capacity  connected  with  C 
by  a  wire — as  shown  by  the  continuous  lines — and  by  a 
string  wetted  with  a  solution  of  chloride  of  calcium,  as 
shown  by  the  dotted  line.  So  long  as  the  gap  at  B  is 
open  this  little  condenser,  which  is  kept  at  the  same 
potential  as  the  large  condenser  by  means  of  the  wire 
and  wet  string,  is  similarly  unable  to  make  sparks  both 
at  B  and  E',  but  it  could,  if  B  was  closed,  at  once  dis- 
charge at  E'.  Now,"  suppose  the  bullet  to  join  the 
wires  at  B,  a  minute  spark  is  made  at  B  and  at  E'  by 
the  discharge  of  c.  Immediately  C,  finding  one  of  its 
gaps,  E',  in  a  conducting  state,  discharges  at  E,  making 
a  brilliant  spark  which  casts  a  shadow  of  the  bullet 
upon  the  photographic  plate,  P.  The  wet  string  suf- 
fices to  charge  the  jar  c,  but  acts  like  an  insulator 
when  the  discharge  takes  place  at  E'  and  B.  The 
photograph  is  a  silhouette,  but  it  serves  to  define  the 
wave  of  air  caused  by  the  bullet. 

Prof.  Boys  remarks  that  the  wave  revealed  by  the 
photograph  shows  just  as  in  the  case  of  waves  produced 
by  the  motion  of  a  ship,  which  become  enormously  more 
energetic  as  the  velocity  increases,  and  which  at  high 
velocities  produce  an  effect  of  resistance  to  the  motion  of 
the  ship  far  greater  than  that  of  skin  friction,  that  the 
resistance  which  the  bullet  meets  increases  very  rapidly 
when  the  velocity  is  raised  beyond  the  point  at  which 
these  waves  begin  to  be  formed.  Scott  Kussell  has 
shown  by  diagrams  and  experiments  what  happens 
when  a  solitary  wave  meets  a  vertical  wall.  As  long  as 
the  wave  makes  an  angle  with  the  wall  it  is  reflected 
perfectly,  making  an  angle  of  incidence  equal  to  the 
angle  of  reflection,  and  the  reflected  and  incident  waves 


218  WHAT  IS  ELECTRICITY  I 

are  alike  in  all  its  parts.  When  the  wave  front  nearly 
perpendicular  to  the  wall  runs  along  nearly  parallel  to 
it,  it  then  ceases  to  be  reflected  at  all.  The  part  of  the 
wave  near  the  wall  gathers  strength;  it  gets  higher, 
travels  faster,  and  so  causes  the  wave  near  the  wall  to 
run  ahead  of  its  proper  position,  producing  a  bend  in 
the  wave  front,  and  this  goes  on  until  the  wave  near 

the  wall  becomes  a 
breaker.  To  see  if  a 
similar  phenomenon 
could  be  traced  in  the 
air,  Prof.  Boys  ar- 
ranged three  reflect- 
ing surfaces  (as  seen 
in  Fig.  31).  Below 
the  bullet  two  waves 
strike  a  reflector  at  a 
low  angle,  and  they 
are  perfectly  reflected. 

The  left  side  of  the  Y-shaped  reflector  was  met  at  nearly 
grazing  incidence.  There  is  no  reflection,  but  the  wave 
near  this  reflector  is  of  greater  intensity ;  it  has  bent 
itself  ahead  of  its  proper  position,  just  as  the  water  wave 
was  found  to  do.  The  stern  wave  has  a  piece  cut  out 
of  it  and  bent  up  at  the  same  angle.  Prof.  Boys  points 
out  that  if  the  wave  was  a  mere  advancing  thing  the 
end  of  the  bent-up  piece  would  leave  off  suddenly,  and 
the  break  in  the  direct  wave  would  do  the  same.  But 
according  to  Huyghens's  hypothesis,  the  wave  at  any 
epoch  is  the  resultant  of  all  the  disturbances  which 
have  started  from  all  points  of  the  wave  front  at  any 
preceding  epoch.  The  reflector,  where  it  has  cut  this 
wave,  may  be  considered  as  a  series  of  points  of  dis- 
turbance arranged  continuously  on  a  line,  each  coming 


WAVE  MOTION.  219 

into  operation  just  after  the  neighbour  on  one  side  and 
just  before  the  neighbour  on  the  other.  The  reflected 
wave  is  the  envelope  of  a  series  of  spheres  beginning 
with  a  point  at  the  place  where  the  wave  and  the  re- 
flector cut,  growing  up  to  a  finite  sphere  about  the  end 
of  the  reflector  to  a  centre ;  beyond  this  there  are  no 
more  centres  of  disturbance,  the  envelope  of  all  the 
spheres  projected  upon  the  plate — that  is,  the  photo- 
graph of  the  reflected  wave — is  not  therefore  a  straight 
line,  leaving  off  abruptly,  but  it  curls  round,  dying 
gradually  to  nothing.  In  the  nonreflection  of  the  air 
wave  by  the  Y-shaped  reflector  we  have  optical  evi- 
dence of  what  goes  on  in  a  whispering  gallery.  The 
sound  is  probably  not  reflected  at  all,  but  runs  round 
almost  on  the  surface  of  the  wall  from  one  part  to 
another. 

A  most  interesting  method  of  studying  sound  waves 
in  air  by  means  of  the  electric  spark  was  devised  by 
Topler.*  He  succeeded  in  making  visible  the  reflection 
and  the  refraction  of  sound  waves,  and  also  the  inter- 
ference of  two  sound  waves. 

An  idea  of  Topler's  method  of  rendering  visible  to 
the  eye  the  waves  of  sound  in  the  air  can  be  obtained 
from  a  consideration  of  the  phenomenon  of  mirage.  A 
low -lying  strata  of  air  of  suitable  density  enables  us  to 
see  objects  below  the  horizon,  for  the  rays  of  light 
(Fig.  32)  from  these  objects  are  bent  or  refracted  to 
the  eye  by  the  strata  of  air.  For  instance,  if  A  repre- 
sents the  position  of  the  horizon,  and  S  that  of  the  sun, 
which  is  a  little  below  the  horizon,  the  strata  of  air 
lying  above  A  can  refract  the  ray  S  C  to  E,  and  look- 
ing along  C  E  we  shall  see  the  sun  apparently  elevated 

*  Annalen  der  Physik  und  Chemie,  131, 1867,  p.  180. 


220  WHAT  IS  ELECTRICITY! 

above  the  horizon  to  D.  If,  now,  a  sufficiently  power- 
ful sound  wave  could  be  generated  on  the  horizon  near 
A,  the  alternate  condensations  and  rarefactions  pro- 
duced by  the  wave  as  it  progressed  upward  in  the  at- 
mosphere would  have  the  effect 
of  producing  images  of  the  sun 
at  points  D,  D',  D2,  etc.,  and 
the  eye  at  E  would  see  in  these 
images  the  progress  of  the  wave 
through  the  air. 

Of  course  we  could  not  pro- 
duce a  sufficiently  powerful 
sound  wave  to  produce  a  mirage  effect,  and  to  thus 
imprint,  so  to  speak,  the  condensation  effects  of  the 
sound  wave  on  the  sky.  The  experiment,  however, 
can  be  performed  in  the  laboratory  with  an  electric 
spark  in  place  of  the  sun,  and  with  a  diaphragm,  A  B, 
instead  of  the  earth.  A  certain  arrangement  of  lenses 
also  serves  to  make  the  phenomenon  more  definite. 

The  ease  with  which  air  transmits  even  a  whisper  is 
wonderful.  The  waves  of  sound  pass  through  doors, 
are  reflected  from  walls,  and  spread  around  innumerable 
obstacles,  and  still  reach  our  ears.  One  is  astonished 
at  the  readiness  with  which  sound  waves  are  reflected 
from  tree-covered  hillsides,  and  at  the  accuracy  of  the 
echo  we  hear.  If  the  eye  could  follow  a  sound  wave 
in  the  air  as  it  passed  in  front  of  one,  one  would  see 
spaces  of  compressed  air  and  spaces  of  rarefied  air. 
Although  we  can  not  see  such  spaces,  we  can  represent 
the  propagation  of  a  sound  wave  so  that  its  motion  at 
any  instant  can  be  studied.  Let  the  intervals  of  time 
be  laid  off  along  M  X  (Fig.  15,  page  143),  and  the  amount 
of  compression  or  rarefaction  of  the  atmosphere  at  each 
interval  of  time  be  laid  off  perpendicular  to  this  line. 


WAVE   MOTION. 


221 


FIG.  33. 


The  varying  compression  will  be  expressed  by  the  curve 
above  the  line  M  X,  and  the  rarefaction  by  the  portion 
below  that  line.  The  curve 
M  a  b  c  d  e  will  then  rep- 
resent a  sound  wave.  By 
an  indirect  process  we  can 
perceive  the  sound  waves 
in  air.  A  small  air  cham- 
ber, (Fig.  33),  is  pro- 
vided on  one  side  with  a 
thin  membrane,  M ;  a  lit- 
tle orifice,  O,  fits  into  this 
air  chamber,  and  gas  is  led 
by  the  pipe  to  this  cham- 
ber. With  a  suitable  gas 

pressure  and  a  suitable  flame  at  O  one  can  study  the 
to-and-fro  motions  of  the  air  at  M  by  means  of  the 
motions  of  the  gas  flame  seen  in  a  revolving  mirror,  K. 
A  modification  of  the  experiment  of  using  parabolic 
mirrors  for  the  transmission  of  sound  waves  is  interest- 
ing to  show  how  extremely  short  sound  waves  may  be 
detected.  At  the  focus  of  the  receiving  mirror  is 
placed  a  small  box,  the  cover  of  which  is  replaced  by  a 
layer  of  thin  bladder.  Gas  under  suitable  pressure  is 
led  by  a  small  tube  into  this  box,  and  a  very  fine  orifice 
opposite  the  leading-in  pipe  is  provided.  On  lighting 
the  gas  at  this  orifice  we  obtain  a  little  flame  which  is 
extremely  sensitive  to  high  notes.  On  placing  the  box 
at  the  focus  of  the  receiving  mirror  and  shaking  a 
bundle  of  keys  or  striking  two  pieces  of  metal  together 
at  the  focus  of  the  sending  mirror  the  flame  will 
lengthen  and  shorten.  Experiments  like  this  help  us 
to  realize  how  the  air  is  filled  with  invisible  waves  of 
sound  when  we  speak,  and  how  they  are  reflected, 


222  WHAT  IS  ELECTRICITY  1 

spread  behind  obstacles,  and  brought  to  a  focus  by 
mirrors.  "We  must,  however,  constantly  bear  in  mind 
the  fact  that  sound  waves  move  to  and  fro  along  the 
direction  in  which  they  are  propagated.  It  is  thus 
only  that  ordinary  air  can  transmit  vibrations.  It  is 
not  capable  of  transmitting  transverse  vibrations,  or,  in 
other  words,  vibrations  at  right  angles,  to  the  direction 
of  propagation.  It  can  not  transmit,  therefore,  light 
and  heat  and  electrical  vibrations  ;  they  apparently  re- 
quire an  ethereal  medium  for  this. 

We  have  seen  that  sound  waves  can  be  represented 
by  sinuous  curves  which  resemble  waves  in  water  or 
waves  in  the  ether.  We  have  said  that  the  atmosphere 
can  not  transmit  transverse  vibrations,  and  that  the  to- 
and-fro  movements  in  the  sound  wave  are  in  the  direc- 
tion of  its  propagation.  ~Now  we  should  expect  that 
the  velocity  of  this  to-and-f ro  movement  would  be 
greater  in  metals  and  liquids  than  in  the  air,  since  in 
solids  and  liquids  the  particles  are  nearer  together  and 
more  numerous  than  in  the  atmosphere.  A  push  would 
therefore  be  transmitted  faster  in  the  denser  media. 
This  is  found  to  be  the  case.  The  velocity  of  sound  in 
iron  is  ten  or  twelve  times  and  in  water  four  or  five 
tunes  that  in  air.  The  light  and  heat  rays,  however, 
travel  slower  in  water  than  in  air.  The  ether  move- 
ments are  impeded  by  the  particles  of  gross  matter. 

The  greater  ease  with  which  sound  travels  in  water 
than  in  air  has  led  to  many  attempts  to  signal  great  dis- 
tances under  water.  ~No  great  measure  of  success  has 
yet  been  obtained  in  this  direction.  By  means  of  a 
microphonic  attachment  to  a  vibrating  diaphragm  I  suc- 
ceeded in  hearing  the  clicking  of  two  stones  under 
water  a  distance  of  six  hundred  feet. 

The  simplest  idea  of  wave  motion  which  is  charac- 


WAVE  MOTION.  223 

teristic  of  light  and  heat  waves  can  be  obtained  from 
the  waves  which  are  propagated  in  water  when  a  stone 
is  thrown  into  a  pond.  The  ripples  extend  in  constantly 
widening  circles  from  the  centre  of  disturbance.  They 
pass  under  bits  of  wood  or  cork,  but  do  not  urge  these 
onward.  They  give  an  up-and-down  motion  to  these 
bits  of  matter.  In  other  words,  the  up-and-down 
motion  of  the  waves  is  transverse  to  the  direction  of 
propagation  of  these  waves.  In  this  respect  the  waves 
in  water  resemble  light  and  heat  waves ;  for  in  these 
waves  also  the  vibration  is  at  right  angles  to  the  direc- 
tion of  propagation.  If  now  we  should  be  in  a  row- 
boat  between  the  wakes  of  two  steamboats,  it  is  evident 
that  if  the  wave  from  one  steamboat  should  tend  to 
urge  our  boat  upward  while  the  wave  from  the  other 
should  tend  to  depress  it,  we  should  escape  being 
swamped,  for  the  two  waves  would  neutralize  each 
other.  On  the  contrary,  if  they  should  both  combine 
to  lift  and  to  depress  our  boat,  the  danger  would  be 
greatly  increased.  There  would  be  points  of  interfer- 
ence of  waves  and  augmentation  of  waves. 

In  the  subject  of  light  and  heat  we  are  dealing  with 
minute  waves ;  and  in  order  to  observe  the  interferences 
of  such  minute  waves  we  must  use  very  small  slits  or 
orifices.  If,  for  instance,  we  should  look  at  the  light  of 
a  candle  through  a  slit  in  a  card  one  tenth  of  an  inch 
wide,  we  can  scarcely  perceive  any  phenomenon  of  inter- 
ference. On  the  other  hand,  if  we  should  cut  a  mere 
line  in  a  thin  piece  of  metal  and  look  at  the  candle,  we 
should  perceive  bright  and  dark  lines  extending  each 
side  of  the  narrow  slit.  These  bright  and  dark  spaces 
are  due  to  the  waves  of  light  which  emanate  from  the 
illuminated  edges  of  the  slit,  and  which  interfere.  In 
looking  through  an  umbrella  at  a  distant  electric  light 


224  WHAT  IS  ELECTRICITY? 

we  can  also  perceive  phenomena  of  the  interference  of 
light  in  passing  through  the  meshes  of  the  covering  of 
the  umbrella. 

Now,  these  waves  in  water  and  waves  of  light  and 
heat  in  the  ether  differ  essentially  from  the  waves  of 
sound.  The  atmosphere  can  not  transmit  transverse 
vibrations,  such  as  we  perceive  in  the  ether.  It  can 
only  transmit  vibrations  which  are  in  the  line  of  the 
onward  movement  of  the  wave.  In  the  movement  of  a 
sound  wave  the  air  is  alternately  compressed  and  rare- 
fied. It  is  beating  like  the  human  heart.  Now  in  this 
peculiarity  of  air — that  it  can  not  transmit  transverse 
vibrations — we  have  one  of  the  strongest  arguments 
for  the  existence  of  the  ether  medium ;  for  we  can 
prove  that  the  vibrations  producing  light  and  heat  are 
transverse  to  the  direction  of  onward  movement  of  the 
waves  of  light  and  heat. 

The  difficulties  which  waves  of  light  and  heat  find 
in  passing  through  liquid  and  crystalline  substances  can 
be  explained  by  the  effect  of  molecular  arrangements 
in  modifying  the  waves  in  the  ether  surrounding  or 
interpenetrating  such  molecular  groupings.  For  in- 
stance, a  plate  of  the  crystalline  tourmaline  will  allow 
only  such  waves  in  the  ether  to  pass  through  it  whose 
transverse  vibrations  are  confined  to  one  plane.  Its 
molecular  grouping  cuts  off  certain  transverse  vibra- 
tions. This  grouping  behaves  to  the  waves,  as  we  have 
already  said  (page  80),  like  the  slats  of  a  Venetian 
blind  to  a  shower  of  stones  which  is  thrown  against  it. 
Only  the  stones  which  are  in  the  plane  of  the  openings 
in  the  blind  can  pass  through.  The  light  waves  are 
then  said  to  be  polarized ;  their  vibrations  are  confined 
to  one  plane.  If  a  ray  of  light  which  has  thus  passed 
through  one  plate  of  tourmaline  should  be  examined  by 


WAVE  MOTION.  225 

another  plate  of  tourmaline,  it  will  be  found  no  light 
can  be  seen  if  the  second  plate  is  turned  into  a  certain 
position.  A  second  Venetian  blind  has  been  turned 
so  that  its  slats  are  at  right  angles  to  the  first,  and  the 
stones  which  passed  through  the  first  blind  are  inter- 
cepted by  the  second.  Thus,  two  plates  of  a  crystal  which 
are  separately  transparent  can  completely  stop  light  from 
passing  through  them  if  combined  in  the  manner  sug- 
gested. Sir  John  Herschel  suggested  that  by  means  of 
this  phenomenon  one  could  telegraph  through  the  air 
secretly.  A  beam  of  light  could  be  transmitted  through 
a  plate  of  some  substance  like  tourmaline,  and  be  thus 
polarized.  The  distant  observer,  provided  with  tourma- 
line spectacles,  could  thus  read  the  signals  which  would 
escape  the  observation  of  every  one  else.  There  are 
many  substances  besides  tourmaline  which  possess  the 
property  of  polarizing  light,  or,  in  other  words,  con- 
fining its  wave  motions  in  one  direction  or  in  one 
plane.  When  we  study  the  subject  of  the  polarization 
of  light,  we  are  led  to  a  consideration  of  the  effect  of 
molecular  groupings  upon  the  transmission  of  the  waves 
of  light  or  heat. 

In  this  chapter,  however,  let  us  confine  ourselves  to 
the  consideration  of  wave  motion,  and  avoid,  for  the 
present,  a  consideration  of  the  entangling  effect  of 
atoms  and  molecules.  Whether  we  accept  the  doctrine 
of  the  ether  or  not,  we  certainly  see  in  the  phenomena 
of  light  and  heat  evidence  of  a  to-and-fro  movement 
which  is  periodic — that  is,  it  is  like  the  motion  of  a 
point  on  the  rim  of  a  bicycle  wheel :  it  rises  to  its  highest 
point  above  the  hub,  and  sinks  to  its  lowest  point  below 
the  hub,  and  goes  through  all  the  intermediate  positions 
with  every  revolution  of  the  wheel.  Its  greatest  am- 
plitude of  movement  above  and  below  the  horizontal  line 


226  WHAT  IS  ELECTRICITY! 

passing  through  the  hub  is  the  radius  of  the  wheel.  It 
goes  through  all  its  points  in  a  certain  period  of  time.  To 
the  eye  standing  behind  the  revolving  wheel  the  motion 
of  the  point  is  like  that  of  a  boat  which  rises  and  falls 
with  a  wave.  If  the  height  of  a  boat  should  be  plotted 
at  progressive  intervals  of  time  while  the  wave  passes 
under  the  boat,  we  should  again 
obtain  a  periodic  curve — periodic 
because  it  recurs  in  the  same 
form.  The  length  of  the  wave 
includes  a  crest  and  a  trough 
(Fig.  34).  If  now  our  boat  should 
be  in  the  neighbourhood  of  a  wall 
of  rock,  at  a  point,  M,  where  the 
incoming  wave  and  the  wave  re- 
flected from  the  rock  meet  (Fig. 
IG<  '  34),  it  would  evidently  not  rise 

or  fall.  This  point  is  called  a  node.  It  is  a  place 
where  there  is  no  motion.  The  other  points  on  the 
wave  belong  to  what  are  called  ventral  segments.  They 
exist  in  the  ether  when  a  light  wave  meets  in  its  prog- 
ress to  a  reflecting  surface  the  backcoming  reflected 
wave.  Since  the  lengths  of  the  light  waves  are  so  ex- 
tremely small — about  ^5-^5^  of  an  inch — it  is  evident 
that  it  would  be  extremely  difficult  to  detect  these  nodal 
points  in  the  ether.*  With  sound  waves,  however, 


*  They  have,  however,  been  photographed  lately  by  Wiener 
(Annalen  der  Physik  und  Chemie,  No.  40,  1890,  p.  203),  and  a  short 
consideration  of  his  process  will  illustrate  the  formation  of  waves. 
Since  the  light  waves  are  so  extremely  small,  the  thickness  of  a 
photographic  film  which  could  show  the  paths  of  the  waves  must  be 
comparable  with  the  wave  length  of  the  light.  Wiener  therefore 
used  a  film  of  collodion,  sensitized  by  a  chloride-of-silver  solution. 
The  thickness  of  the  film  was  about  2V  to  -fa  of  the  wave  length  of 


WAVE  MOTION.  227 

which  are  several  feet  long,  the  nodal  points  can  be 
readily  made  manifest.  We  shall  see  that  apparently 
they  can  also  be  detected  when  electro-magnetic  waves 
are  reflected  from  the  walls  of  a  room. 

Perhaps  the  most  striking  evidence  that  the  phe- 
nomena of  light  and  heat  are  due  to  wave  motions  is 
obtained  from  the  phenomena  of  interference  of  waves ; 
and  since  investigators  are  beginning  to  speak  of  an 
electrical  spectrum,  it  is  well  to  obtain  an  idea  of  the 
light  and  heat  spectrum,  which  is  really,  according  to 
our  modern  ideas,  also  an  electrical  spectrum. 

If  two  pieces  of  perfectly  plane  glass  are  placed 
upon  each  other  so  as  to  include  a  thin  film  of  air  be- 
tween their  plane  surfaces,  beautiful  bands  of  colour  are 
seen  when  these  surfaces  are  looked  at  obliquely.  These 
colours  are  due  to  the  interference  of  waves  of  light,  and 
are  produced  in  the  same  manner  as  the  bands  of  colour 
on  soap  bubbles  or  the  colours  of  thin  films  of  oil  on 
water.  If  a  cent  which  has  been  slightly  warmed  in 
the  fingers  is  placed  on  the  upper  surface  of  the  upper 
glass  plate,  the  coloured  bands  immediately  shift  their 
position  and  change  from  straight  bands  to  curved  ones. 
This  effect  is  produced  by  the  expansion  of  the  glass 


sodium  light  (about  SWOTT  of  an  inch).  The  light  was  reflected  per- 
pendicularly from  a  mirror.  If  the  sensitive  plate  was  placed  per- 
pendicular to  the  surface  of  the  mirror,  it  is  evident  that  the  nodal 
points  corresponding  to  M,  Fig.  34,  where  the  reflected  wave  crossed, 
so  to  speak,  the  incident  wave,  would  lie  in  a  series  of  planes  par- 
allel to  the  mirror.  The  distances,  however,  between  such  nodal 
points  would  be  too  small  to  observe  on  the  photograph.  If,  now,  the 
photographic  plane  is  inclined  at  a  small  angle  to  the  mirror,  the 
apparent  distances  between  the  nodes  is  increased,  and  a  series  of 
dark  bands  can  be  obtained  on  the  sensitive  plate  which  show  the 
nodal  points  and  the  neutral  segments  of  the  wave.  In  this  way 
photographs  of  light  waves  are  obtained. 
1G 


228  WHAT  IS  ELECTRICITY! 

due  to  the  heat  of  the  cent,  and  this  expansion  produces 
a  variation  in  the  thickness  of  the  thin  film  of  air  be- 
tween the  glasses.  This  effect  can  be  observed  with  glass 
plates  half  an  inch  in  thickness. 

The  interference  of  the  waves  of  light  is  also  illus- 
trated by  Lippman's  process  of  colour  photography. 

Lippman  states  that  the  essential  conditions  for  ob- 
taining colours  in  photography  are  two :  First,  con- 
tinuity of  the  sensitive  layer ;  second,  presence  of  a  re- 
flecting surface  against  which  the  sensitive  plate  rests. 
By  continuity  of  layer  is  meant  the  absence  of  grains. 
It  is  necessary  that  the  iodide  or  the  bromide  of  silver 
should  be  disseminated  throughout  the  mass  of  a  plate  of 
albumen  or  gelatine  in  a  uniform  manner  without 
forming  grains  which  are  of  sensible  size  with  reference 
to  the  dimensions  of  the  waves  of  light.  Lippman  em- 
ploys a  support  of  albumen,  collodion,  or  gelatine  with 
the  iodides  and  bromides  of  silver.  The  dry  plate  is 
carried  in  a  tray  in  which  one  pours  mercury.  This 
mercury  forms  a  reflecting  surface  in  contact  with  the 
sensitive  plate.  The  exposure,  development,  and  fix- 
ing is  the  same  as  in  the  ordinary  process,  but  when  the 
plate  is  fixed  and  dried  colours  appear.  The  plate  is 
negative  by  transmitted  light  and  coloured  by  reflected 
light.  The  theory  is  as  follows :  The  incident  light 
which  forms  the  image  interferes  with  the  light  reflected 
from  the  surface  of  the  mercury.  There  are  therefore 
formed  a  system  of  interference  bands — maxima  and 
minima — in  the  interior  of  the  sensitive  layer.  The 
maxima  alone  are  impressed  upon  the  layer,  and  remain 
marked  by  layers  of  silver  more  or  less  reflecting  which 
take  their  place.  The  sensitive  layer  is  thus  divided  by 
these  layers  into  a  series  of  thin  plates,  the  thickness  of 
which  is  equal  to  the  interval  which  separates  two 


WAVE  MOTION.  229 

maxima — that  is  to  say,  to  a  half  wave  length  of  the  in- 
cident light.  These  thin  layers  have  therefore  precise- 
ly the  necessary  thickness  to  reproduce  the  incident 
light  by  reflection.  The  visible  colours  are  thus  of  the 
nature  of  those  one  sees  on  soap  bubbles.  They  are, 
however,  purer  and  more  brilliant  if  the  photographic 
operations  have  given  a  good  reflecting  surface ;  for  one 
forms  in  the  thickness  of  the  sensitive  layer  a  very  great 
number  of  thin  plates  superposed;  about  200  of  the 
layers  amount  to  fa  of  a  millimetre.  The  reflected  col- 
our is  the  purer  the  greater  the  number  of  reflecting 
surfaces  (Comptes  Rendus,  112,  1891).  Lippman  also 
applied  the  orthochromatic  process  to  his  method.  With 
layers  of  albumen,  sensitized  by  bromide  of  silver,  ren- 
dered orthochromatic  by  azurine  and  cyanine,  he  ob- 
tained brilliant  photographs  of  the  solar  spectrum.  He 
exhibited  to  the  French  Academy  a  photograph  of  a 
stained  window  with  four  colours — red,  green,  blue,  and 
yellow ;  a  group  of  dra-peaux,  a  plate  of  oranges  sur- 
mounted by  a  paroquet.  The  drapeaux  and  the  bird 
were  exposed  five  to  ten  minutes  under  the  electric 
light ;  the  other  objects  were  exposed  several  hours  to 
different  light.  It  is  necessary,  therefore,  to  greatly  in- 
crease the  sensitiveness  of  the  plates. 

Twenty  years  ago  Robert  succeeded  hi  ruling  fine 
lines  on  glass  which  were  separated  by  as  small  a  dis- 
tance as  i  b  ^  0  0  of  an  inch.  His  lines  were  used  as  a 
test  of  the  resolving  power  of  microscopes,  and  his 
method  of  producing  them  was  considered  a  secret.  It 
was  not  long,  however,  before  it  was  discovered  that  a 
diamond  cutter  could  be  moved  by  means  of  an  accu- 
rately cut  screw  through  very  small  spaces.  Thus,  if 
the  distance  between  the  threads  of  the  screw  is  ^  of 
an  inch,  and  a  circle  f  orming  one  end  of  the  screw  were- 


230  WHAT  IS  ELECTRICITY! 

divided  into  one  thousand  parts,  the  movement  of  the 
head  through  one  of  its  smallest  divisions  would  move 
the  diamond  cutter  connected  with  a  nut  on  the  screw 
through  ssooo  of  an  inch.  At  one  point  a  line  could  be 
cut  and  a  parallel  line  2  s  1 0  0  of  an  inch  from  this,  and 
so  on.  Success  in  forming  a  good  diffraction  grating — 
for  so  this  assemblages  of  fine  lines  was  called — consisted 
merely  in  using  an  accurate  screw  and  a  plane  surface. 

If  a  source  of  light  (Fig.  35)  is  placed  at  D,  the  rajs 
reflected  by  the  sides  of  the  diamond  cuts  B  and  C 
(greatly  magnified  in  the  drawing)  will  pursue  paths  A 
C  and  A  B,  which  may  differ  by  an 
odd  number  of  half  wave  lengths,  in 
D  s  which  case  they  will  neutralize  each 

\p^  other  at  A,  and  there  will  be  no  light, 

or  else  they  may  differ  by  an  even 
number  of  wave  lengths,  in  which  case 
there  will  be  light  at  A.  We  shall 
thus  have  a  spectra  interspersed  with 
dark  spaces  along  a  line  passing 
x '  through  A.  Instead  of  using  lines 
ruled  by  a  diamond  on  a  glass  plate 
and  transmitted  light,  we  can  employ  such  lines  on  spec- 
ulum metal,  and  employ  reflected  light ;  in  which  case 
the  light  appears  to  come  from  a  point  behind  tho 
mirror,  as  shown  in  the  lower  part  of  Fig.  35.  Another 
way  to  produce  interference  is  due  to  Prof.  Michelson. 
He  has  made  two  important  uses  of  his  apparatus. 

A  source  of  light,  S  (Fig.  36),  is  reflected  by  a  plane 
parallel  glass,  B,  to  a  plane  mirror  at  A.  The  latter  in 
turn  refracts  it  through  the  glass  to  D.  Another  beam 
is  reflected  to  C,  and  then  back  to  D.  There  is  there- 
fore a  difference  in  the  paths  of  the  rays  reflected  from 
A  and  C,  and  interference  takes  place  at  D  in  the  shape 


WAVE  MOTION.  231 

of  a  great  number  of  fine  dark  lines.  Prof.  Michelson 
used  a  modification  of  this  apparatus  to  test  the  ques- 
tion whether  the  ether  moves  with  the  earth.  The  ap- 
paratus was  set  up  in  such  a  way  that  the  path  of  the 
light  A  C  was  in  the  direction  of  the  movement  of  the 
earth.  The  mirror  A  therefore  is  carried  to  A',  while 
the  reflected  light  travels  to  B  and  returns.  If  the 
ether  did  not  move  with  the  earth  the  position  of  in- 
terference bands  would  indicate  this.  The  apparatus 
was  set  up  with  the  greatest  care  at  Potsdam,  near  Ber- 
lin, but  no  effect  could  be  noticed.  Prof.  Michelson 
has  lately,  by  means  of  this  method,  based  the  meas- 
urement of  a  standard  of  length  on  the  measurement  of 
the  wave  length  of  light.  If  the 
paths  B  A  or  A  C  are  altered,  this 
change  can  be  measured  by  the 
number  of  interference  bands 
which  cross  the  field  of  view  at 
D.  The  change  in  length  can  thus 
be  estimated  in  a  definite  number 
of  wave  lengths  of  light.  The  Pia'  36' 

tenth  of  a  metre  (or  the  decimetre)  can  thus  be  ex- 
pressed in  wave  lengths.  The  advantage  of  this  method 
of  measurement  consists  in  using  an  unchangeable 
standard  instead  of  the  length  of  King  Henry's  arm. 
The  imperial  yard  or  metre  is  subject  to  expansion  and 
contraction,  and  doubtless  to  secular  change.  Prof. 
Michelson' s  standard  is  apparently  immutable. 

The  Rowland  concave  grating  is  the  greatest  con- 
tribution that  has  been  made  to  the  subject  of  spectrum 
analysis  since  Rutherford  showed  the  possibility  of 
making  diffraction  gratings  which  would  give  spectra 
of  great  brilliancy  and  large  dispersion.  Instead  of 
using  a  plane  surface  ruled  with  fine  lines,  Rowland 


232 


WHAT  IS  ELECTRICITY! 


FIG.  37. 


employs  the  surface  of  a  concave  mirror  of  long  focus. 
The  waves  of  light  in  striking  these  fine  lines  are  re- 
flected and  brought  to  a  focus  by  the  concave  mirror. 
The  modern  spectroscope  forms  a 
striking  contrast  to  the  large  spec- 
troscopes which  were  used  twenty 
years  ago.  In  the  latter  form  the 
light  was  sent  through  a  great 
number  of  prisms  to  refract  and 
disperse  it  to  the  utmost.  In  some 
cases  the  light  after  being  re- 
fracted by  one  set  of  prisms  (Fig. 
37)  was  sent  up  one  story  by  means  of  plane  mirrors, 
and  was  then  sent  round  another  set  of  prisms.  It  is 
evident  that  the  materials  of  the  prisms  modified  to  a 
great  extent  the  amount  and  character  of  the  dispersion 
obtained.  The  modern  spectroscope  (Fig.  38)  consists 
simply  of  an  illuminated  slit,  S,  a  concave  mirror,  C, 
ruled  with  fine  lines,  and  an  eyepiece  or  photographic 
plate  at  E. 

It  is  interesting  to  reflect  that  the  solar  spectrum 
was  before  the  eyes  of  men  like  Sir  Isaac  Newton  and 
Goethe,  with  its  intimations  of  the  inner  mysteries  of 
the  sun  and  stars,  yet  these  mysteries  were  veiled  from 
such  men.  For  many  years  after 
Bunsen  and  Kirchhoff  pointed 
out  the  road  to  spectrum  analysis 
the  spectrum  was  the  subject  of 
human  inquiry,  yet  it  contained 
other  mysteries  the  clews  to  which 
have  been  slowly  grasped  even 
by  the  best  minds.  The  fact  that  the  distinction  be- 
tween light  rays  and  heat  rays  resides  only  in  a  differ- 
ence of  wave  length  has  slowly  dawned  upon  human 


FIG.  38. 


WAVE  MOTION.  233 

intelligence.  It  was  not  suspected  twenty  years  ago 
that  the  spectrum  of  the  invisible  rays  which  we  call 
heat  rays  extends  to  a  length  exceeding  the  visible 
spectrum  ten  or  twelve  times. 

To  the  labours  of  Prof.  Langley  we  are  indebted  to 
a  great  extension  of  our  knowledge  of  the  invisible  heat 
rays.  The  method  he  employed  to  detect  these  rays  is 
an  interesting  example  of  the  transformation  of  energy. 
The  instrument  by  means  of  which  he  opened  up  this 
undiscovered  country  he  terms  a  bolometer — from  /9o\?;, 
throw,  and  perpov,  a  measure.  It  consists  of  a  very  h'ne 
wire  through  which  circulates  a  steady  current  of  elec- 
tricity. A  very  slight  change  in  temperature  of  this 
fine  wire  will  cause  a  change  in  its  resistance,  and  a 
quick  movement  of  a  galvanome- 
ter needle  connected  with  this 
wire  will  result.  This  throw  of 
the  galvanometer  needle  is  taken 
as  a  measure  of  the  amount  of 
heat  which  falls  on  the  fine  wire 
when  the  spectrum  is  moved  ^_, 

across  such  a  wire.  The  galva- 
nometer connected  with  the  wire  will  show  a  cooling 
effect  when  a  dark  solar  line  passes  across  the  wire. 
In  this  way  Prof.  Langley  has  detected  and  mapped 
thousands  of  lines  in  the  invisible  red,  where  only  a 
dozen  were  known  to  exist  before  his  labours. 

I  have  said  that  the  bolometer  is  an  interesting  ex- 
ample of  the  transformation  of  energy.  While  it  is 
being  heated  by  the  electric  current  passing  through  it, 
it  is  so  sensitive  to  the  temperature  of  outside  bodies 
that  it  serves  as  a  measure  of  the  energy  it  receives 
from  these  bodies.  A  heat  wave  may  be  said  to  be 
transformed  into  an  electric  current  to  register  itself. 


234  WHAT  IS  ELECTRICITY? 

By  the  aid  of  the  bolometer  Langley  has  been  enabled 
to  measure  the  heat  of  the  moon  and  to  examine  its 
spectrum.  The  actual  ar- 
rangement of  the  bolom- 
eter apparatus  is  that  of 
the  Wheatstones  bridge 
(Fig.  40). 

The  action  of  a  fine 

wire  which  is  traversed 
FIG.  40.  _     j  . 

by   an    electric    current 

toward  radiant  heat  is  very  analogous  to  that  of  the 
carbon  transmitter  toward  sound  waves.  If  we  connect 
two  carbon  points  (Fig.  39)  in  circuit  with  a  battery 
and  afifix  further  one  of  these  points  to  a  vibrating  dia- 
phragm, we  have  practically  the  modern  carbon  trans- 
mitter. The  voice  changes  the  resistance  at  the  carbon 
points  and  makes  the  electric  current  fluctuate  in  unison 
in  the  distant  telephone.  The  resistance  of  the  bolome- 
ter also  changes  with  the  waves  of  radiant  heat  which 
fall  upon  it.  In  both  the  carbon  transmitter  and  the 
bolometer  the  energy  of  wave  motion  is  changed  into 
electrical  manifestations. 

Can  not  we  therefore  speak  to  a  fine  wire  and  use 
its  fluctuations  of  resistance  to  measure  the  energy  with 
which  we  speak  ?  This  is  possible.  We  have  only  to 
supplant  the  galvanometer,  G,  in  Fig.  40,  by  a  telephone. 
The  fluctuating  currents  produced  in  the  telephone  by 
the  human  voice  disturb  the  distribution  of  the  elec- 
trical currents  through  the  sides  of  the  parallelogram,  of 
which  the  bolometer  wire,  A  D,  makes  one  portion,  but 
how  shall  we  obtain  evidence  of  this  transformation  ? 
One  of  the  best  methods  is  due  to  Profs.  Kubens  and 
Arons,  who  have  modified  the  distribution  of  currents 
in  the  Wheatstones  bridge.  Parallelograms  of  fine 


WAVE  MOTION. 


235 


iron  wire,  W,  and  Ws  (Fig.  41),  take  the  place  of  the 
usual  resistances  between  A  D  and  D  C  (Fig.  40).  If 
one  connects  a  tele- 
phone to  the  points 
B  and  D  (Fig.  41) 
and  speaks  into  it, 
the  distribution  of 
the  currents  along 
the  sides  of  the  lit- 
tle parallelogram  ~Wa 
is  disturbed.  This 
disturbance  leads  to  a 
disturbance  between 
D  and  B  (Fig.  40), 
which  is  made  evi-  pIG.  41. 

dent  by  the  move- 
ment of  the  galvanometer  needle  G.     The  voice  thus 
alters  the  circulation  of  electricity  through  this  net- 
work of  conductors. 

We  have  now  used  the  fluctuations  of  electrical  re- 
sistance to  make  manifest  the  waves  of  heat  and  light 
and  sound.  Can  we  also  use  this  instrument  for  the 

detection  of  electrical 
waves  ?  This  has  been 
accomplished  by  Prof. 
Rubens  and  Prof. 
Arons,  who  supplant- 
ed the  telephone  in 
Fig.  41  by  loops  of 
FIG.  42.  wire,  O  and  P,  on  glass 

tubes  (Fig.  42)  which 

could  be  slipped  along  the  wires  traversed  by  the  elec- 
trical waves.  The  fluctuations  of  electricity  at  electrical 
ventral  segments  and  nodes  thus  caused  corresponding 


236  WHAT  IS  ELECTRICITY? 

disturbances  through  the  glass  tubes  on  the  terminals  O 
and  P,  which  in  turn  disturbed  the  distribution  of  elec- 
trical currents  in  the  parallelogram  W,.  The  little  glass 
tubes  are  really  Lejden  jars.  The  portion  of  the  wire 
in  the  tubes  along  which  the  electrical  waves  are  propa- 
gated constitute  the  inner  coating  of  these  jars,  and  the 
loops  O  and  P  the  outer  coatings. 

In  a  previous  chapter  on  the  rates  of  change  of 
magnetic  induction ;  we  have  shown  how  varied  are 
the  transformations  of  energy  which  can  be  effected  by 
the  quivering,  so  to  speak,  of  lines  of  magnetic  flow. 
In  the  use  of  these  little  Leyden  jars  we  perceive  that 
the  electrostatic  lines  can  also,  by  their  rate  of  change, 
effect  a  rate  of  change  of  magnetic  lines  of  flow.  The 
fluctuation  of  the  electrical  charge  on  the  coatings  of 
the  little  jars  produces  a  current  on  the  wire  connecting 
them.  Indeed,  if  this  fluctuation  is  accompanied  by  a 
sufficiently  powerful  electro-motive  force,  we  have  seen 
that  it  can  produce  the  usual  evidence  of  an  electric 
current — a  spark.  On  the  other  hand,  if  we  should 
speak  into  the  telephone  in  Prof.  Rubens' s  apparatus, 
we  ought  theoretically  to  obtain  an  electrostatic  disturb- 
ance on  the  little  Leyden  jars — that  is,  the  fluctuations 
of  the  magnetic  induction  produced  in  the  telephone  by 
the  human  voice  should  be  transformed  into  fluctua- 
tions of  electrostatic  lines  between  the  coatings  of  the 
Leyden  jars. 

The  effect  would  be  small,  but  it  would  be  possible. 
This  effect,  indeed,  has  been  shown  substantially  by 
Dolbear,  whose  telephone  is  a  Leyden  jar  to  which  one 
listens  while  a  fluctuation  on  the  wires  connecting  its 
two  coatings  is  caused  by  the  human  voice  acting  on  a 
carbon  transmitter  placed  in  the  primary  circuit  of  a 
Ruhmkorff  coil,  while  the  Leyden  jar,  of  which  the  di- 


WAVE  MOTION.  237 

electric  is  air,  is  connected  with  the  terminals  of  the 
secondary  coil  of  the  Ruhmkorff. 

Our  conceptions  of  the  energy  and  rapidity  of  the 
changes  which  can  be  produced  by  fluctuations  of  the 
flow  of  magnetic  induction  and  of  electrostatic  induc- 
tion have  been  greatly  enhanced  by  the  invention  of 
the  telephone.  And  we  are  now,  even  in  practical  ap- 
plications of  electricity,  obtaining  a  realizing  sense  of 
the  importance  of  arranging  our  electrical  circuits  so 
that  the  waves  which  produce  the  fluctuations  of  magnet- 
ic and  electrostatic  induction  should  have  their  proper 
expression  in  the  transformation  we  desire  to  accom- 
plish. The  scientific  imagination  even  looks  forward 
to  transmitting  intelligence  from  America  to  Japan  by 
suitably  modifying  the  electric  charge  on  the  earth. 
This  does  not  seem  at  first  sight  more  improbable  than 
the  feat  of  speaking  by  means  of  the  waves  of  heat, 
which  is  accomplished  by  Graham  Bell's  photophone. 
The  following  interesting 

modification   of   this  in-                                      X 
strument  has  lately  come         ,                                    x  A 
to  my  attention :  /  T 7 


A  tube  of  lampblack, 
T  (Fig.  43),  is  placed  at 
the  focus  of  a  large  para-  FIG.  43. 

bolic  mirror,  and  a  speak- 
ing tube,  or  rather  a  listening  tube,  is  connected  with  it. 
At  the  distance  of  half  a  mile  a  reflecting  mirror  is 
supported  so  that  the  rays  of  the  sun  shall  be  reflected 
to  the  tube,  T.  The  voice  of  a  speaker  at  A  impinging 
against  the  mirror  can  be  thus  heard,  and  conversation 
carried  on  through  the  air.  The  heat  rays  are  thrown 
into  vibration  by  means  of  the  speaker's  voice  upon 
the  lampblack,  and  the  air  in  the  speaking  tube  is 


238  WHAT  IS  ELECTRICITY! 

thus  tuned  to  his  voice.  Here  we  have  a  transforma- 
tion of  sound  movements  into  heat  movements  and  a 
retransfonnation  into  sound  waves.  Can  we  not  trans- 
form the  sound  waves  of  the  voice  into  light  waves,  so 
that  a  speaker  can  cause  light  to  appear  at  a  great  dis- 
tance from  him  ?  To  accomplish  this,  we  need  only  to 
set  the  electrostatic  lines  of  force  to  quivering  in  unison 
with  the  voice. 


CHAPTEE  XYIII. 

ELECTEIO   WAVES. 


we  survey  tlie  practical  development  of 
electricity,  which  I  have  outlined,  we  are  struck  with  the 
fact  that  our  minds  have  been  led  from  a  consideration 
of  steady  currents  of  electricity  and  the  phenomena 
produced  by  them  to  what  may  be  termed  unsteady  or 
periodic  currents.  The  transformations  of  energy  which 
are  possible  with  periodic  or  alternating  currents  are  far 
more  varied  'than  those  we  can  accomplish  with  steady 
currents.  It  would  seem  that  even  the  development  of 
the  applications  of  the  alternating  current  suggests  the 
electro-magnetic  theory  of  light.  The  swifter  the  rate 
of  alternation  of  our  alternating  dynamo,  the  nearer  we 
approach  to  the  manifestations  of  light  and  the  more 
varied  become  the  electrical  phenomena.  This  seems 
to  me  the  most  remarkable  conclusion  to  be  drawn  from 
Tesla's  experiments  on  high  frequency  discharges.  If 
we  could  excite  electrical  currents  which  would  oscillate 
some  billions  of  times  a  second  we  might  produce  the 
sensation  of  light  on  the  retina  of  the  eye  without  a 
spark. 

The  ordinary  Ley  den  jar  is  the  swiftest  alternating 
machine  which  we  can  use  at  present.  Joseph  Henry 
showed  conclusively,  in  1840,  that  the  discharge  of  a 
condenser  is,  in  general,  oscillatory.  His  observations 


240  WHAT  IS  ELECTRICITY! 

on  this  oscillation  form  an  epoch  in  the  study  of  elec- 
tricity, and  the  attention  of  the  scientific  world  is  now 
closely  directed  to  the  manifestations  which  he  discov- 
ered, and  which  Lord  Kelvin  in  1850  expressed  in  a 
mathematical  law  which  forms  the  basis  of  Hertz's  cele- 
brated work. 

The  rate  of  alternation  of  the  Leyden-jar  machine 
depends  upon  the  capacity  of  the  jar  and  the  self-in- 
duction of  the  wire  connecting  its  outer  coating  with 
the  inner  coating.  The  swiftest  rate  of  alternation  we 
can  obtain  from  an  alternating  dynamo  is  barely  one 
hundred  thousand  alternations  per  second,  and  this  rate 
has  practically  not  been  reached.  With  a  Leyden-jar 
discharge  we  can  obtain  and  make  evident  by  photog- 
raphy the  rate  of  ten  millions  per  second.  The  light- 
ning discharge  is  a  discharge  from  the  Leyden  jar 
formed  by  the  layers  of  cloud  and  the  earth,  and  its 
destructive  effect  is  in  part  due  to  its  rapidity  of  oscil- 
lation. 

Now  two  Leyden  jars  with  their  circuits  of  wire  can 
be  electrically  tuned  so  as  to  be  in  unison  with  each 
other,  and  when  one  jar  is  discharged  the  neighbouring 
one,  which  has  not  been  charged,  will  also  give  a  spark 
arising  from  what  is  termed  electrical  resonance. 

It  is  important  that  we  should  obtain  a  clear  idea  of 
what  may  be  termed  electrical  tuning  or  the  obtaining 
of  resonance.  Let  us,  in  the  first  place,  examine  what 
is  termed  resonance  in  the  subject  of  sound.  One  of 
the  simplest  methods  of  showing  acoustical  resonance 
is  to  sound  a  tuning  fork  over  the  mouth  of  a  long  ver- 
tical jar  and  then  slowly  pour  water  into  the  jar.  At 
certain  points  the  column  of  air  in  the  tube  will  vibrate 
in  resonance  with  the  tuning  fork  and  a  great  aug- 
mentation of  sound  results.  The  particles  of  air  swing 


ELECTRIC  WAVES.  241 

in  tune  with  the  prongs  of  the  fork.  The  best  way  of 
showing  this  phenomenon  is  to  immerse  a  large  glass 
tube  open  at  both  ends  in  a  larger  glass  jar  which  is 
filled  with  water.  By  moving  the  inner  glass  tube  up 
or  down  one  can  lengthen  or  shorten  the  column  of  air 
at  pleasure  and  thus  tune  it  to  the  fork.  Another  sim- 
ple method  showing  the  effect  of  resonance  consists 
in  mounting  two  forks  which  give  the  same  note  on 
hollow  boxes  closed  at  one  end  and  open  at  the  other, 
and  placing  these  boxes  with  their  open  ends  close  to- 
gether. If  the  two  forks  are  exactly  in  tune  when  one 
is  excited  by  a  violin  bow  the  other  will  respond,  and 
will  continue  to  vibrate  when  the  exciting  fork  is 
brought  to  rest.  If  the  forks  are  not  in  tune  they  can 
be  brought  into  resonance  by  loading  the  prongs  of  one 
of  the  forks  with  a  little  piece  of  wax.  Notes  suitably 
struck  on  any  stringed  musical  instrument  can  excite 
similar  notes  on  another  stringed  instrument.  Elec- 
trical tuning  or  the  obtaining  of  electrical  resonance 
depends  upon  conditions  analogous  to  those  which  ob- 
tain in  the  subject  of  acoustics,  and  can  be  illustrated 
best  by  considering  the  photographs  of  electric  sparks 
(Plate  I,  frontispiece). 

Let  us  now  endeavour  to  arrange  the  electrical  cir- 
cuits which  shall  be  in  resonance.  It  will  be  necessary 
first  to  obtain  the  time  of  oscillation  of  one  circuit,  and 
then  arrange  another  circuit  which  shall  have  the  same 
time  of  oscillation ;  the  latter  circuit  will  then  be  in 
resonance  with  the  first  circuit.  One  obvious  way  to 
accomplish  this  would  be  to  discharge  a  Leyden  jar 
through  the  circuit  and  photograph  the  spark  which  is 
thus  produced  by  means  of  a  revolving  concave  mirror. 
Knowing  the  speed  of  the  revolving  mirror  and  the 
distance  to  the  sensitive  plate,  we  can  obtain  the  time 


242 


WHAT  IS  ELECTRICITY? 


of  vibration  of  the  circuit,  and  then  we  can  arrange 
another  electrical  circuit  which  will  have  the  same 
number  of  vibrations.  If  these  two  circuits  are  then 
placed  parallel  to  each  other,  even  twenty  feet  apart,  a 
spark  through  one  circuit  will  excite  a  spark  in  the 
other  circuit.  A  simple  way  of  arranging  this  experi- 
ment is  as  follows :  An  electrical  machine  is  employed 
to  charge  a  Leyden  jar,  A  (Fig.  44).  The  accumulated 
charge  equalizes  itself  between  m  and  n  around  the  cir- 


FIG.  44 

cuit  which  connects  n  with  the  outside  of  the  jar.  At 
the  instant  a  spark  passes  between  m  and  n  a  spark  is 
seen  to  jump  between  o  and  p  in  the  circuit  connected 
with  the  jar  B.  As  I  have  said,  these  circuits  can  be 
placed  from  ten  to  twenty  feet  apart,  and  can  be  made 
to  respond  to  each  other.  Two  principal  factors  enter 
into  the  phenomenon  of  electrical  resonance :  the  ar- 
rangement of  wire  in  the  coils  which  are  opposed  to 
each  other,  and  the  number  of  Leyden  jars — or,  in  other 
words,  the  amount  of  capacity  which  is  connected  with 
these  coils.  Thus  in  the  above  experiment  we  have  the 
coils  and  the  Leyden  jars.  The  latter  serve  to  accumu- 


ELECTftIC  WAVES.  243 

late  the  charge  of  electricity  and  to  discharge  it  through 
the  coils. 

When  we  see  an  electric  spark  we  must  reflect  that 
a  magnetic  wave  reaches  our  eyes  at  the  same  instant 
as  the  light.  Its  velocity  in  the  ether  is  the  same  as 
that  of  the  light  rays.  When  a  spark  occurs  in  one 
circuit  a  spark  will  also  occur  in  another  circuit;  it 
may  be  across  the  room,  if  the  latter  circuit  is  parallel 
to  the  first  circuit  and  properly  tuned  to  this  circuit. 
The  energy  of  the  first  spark  is  conveyed  through  the 
ether  in  magnetic  waves  to  the  second  circuit.  The 
second  spark  appears  apparently  at  the  same  instant  as 
the  exciting  spark.  The  velocity  of  propagation  of  the 
magnetic  waves  which  produce  the  spark  is  probably 
the  velocity  of  light.  The  velocity  of  electricity  should 
be  measured  in  free  space,  and  not  on  conductors,  for 
on  metals  its  propagation  is  retarded,  and  it  takes  time 
for  the  current  to  arrive  at  its  greatest  strength.  Thus, 
if  we  have  two  parallel  circuits  with  a  battery  and  key 
in  one  of  the  circuits,  and  if  we  touch  the  key  so  as  to 
send  a  current  along  one  circuit,  we  cause  lines  of  force 
to  spread  out  in  circles  from  the  circuit,  and  these  lines 
cause  a  current  in  the  neighbouring  wire  in  the  opposite 
direction  to  the  exciting  current.  The  circles  of  force 
emanating  from  this  second  circuit  embrace  the  first 
circuit  and  set  up  a  current  in  it  opposed  to  the  excit- 
ing current. 

In  a  paper  on  the  oscillations  of  lightning  dis- 
charges,* I  expressed  the  opinion  that  the  method  first 
employed  by  Spottiswoode,  of  exciting  a  Kuhmkorff 
coil  or  transformer  by  means  of  an  alternating-current 
dynamo,  put  in  the  hands  of  an  experimenter  a  far 


*  Phil.  Mag.,  October,  1893. 
17 


244  WHAT  IS  ELECTRICITY? 

more  powerful  method  of  studying  electrical  oscilla- 
tions than  the  old  method  of  charging  Leyden  jars  by 
means  of  an  electric  machine  or  by  the  use  of  a  Huhm- 
korff  coil  with  a  battery.  I  have  therefore  employed 
an  alternating  machine  capable  of  giving  120  volts  and 
a  current  of  from  15  to  25  amperes,  and  have  em- 
ployed suitable  transformers  to  obtain  the  necessary 
difference  of  potential  to  produce  the  sparks  which  I 
wished  to  study. 

Generally  I  have  employed  one  primary  or  exciting 
circuit  between  two  entirely  separate  and  disconnecting 
resonating  or  secondary  circuits.  The  image  of  the 
three  sparks  thus  produced  could  then  be  compared 
upon  the  same  plate. 

Without  entering  into  a  more  detailed  account  of  the 
apparatus  I  employed,  I  will  state  the  most  striking  re- 
Bults  which  I  have  obtained.  A  unidirectional  spark 
(nonoscillatory)  always  excites  an  oscillatory  discharge 
in  a  secondary  circuit  if  the  self-induction,  capacity, 
and  resistance  of  this  secondary  circuit  permit  an  oscil- 
latory movement.  It  is  therefore  not  necessary  that 
the  spark  in  a  primary  circuit  should  be  an  oscillating 
one  in  order  to  excite  oscillations  in  a  neighbouring 
conductor.  In  this  respect  two  electrical  circuits  are 
not  in  close  analogy  with  two  tuning  forks.  It  is  diffi- 
cult by  a  unidirectional  movement  of  the  prongs  of  one 
tuning  fork  to  excite  the  vibrations  of  another  fork 
which  is  not  in  tune  with  the  first  fork.  In  every  sec- 
ondary circuit,  or  circuits  neighbouring  to  the  primary 
circuit,  the  first  effect  of  the  exciting  unidirectional 
primary  spark  is  to  make  the  .secondary  circuits  act  as 
if  there  were  no  capacity  in  their  circuits.  In  these 
circuits  a  threadlike  spark  results  which  is  exactly  like 
that  produced  when  all  the  capacity  in  the  secondary 


ELECTRIC  WAVES.  245 

circuits  is  removed.  After  a  short  interval  of  time  the 
electricity  rushes  into  the  condensers  and  begins  to 
oscillate,  the  strength  of  the  oscillations  rising,  after 
one  or  two  vibrations,  to  a  maximum  and  then  decreas- 
ing ;  the  rate  of  oscillation  finally  assumes  a  steady 
state.  The  electricity  seems  to  be  separated  only  along 
the  wires  at  first,  and  the  circuit  vibrates  more  like  a 
closed  organ  pipe  than  an  open  one. 

If  a  unidirectional  primary  spark  excites  oscillations 
in  neighbouring  circuits  which  are  slightly  out  of  tune, 
the  phenomenon  of  electrical  beats  or  interferences  can 
be  produced  in  these  circuits,  and  can  be  shown  by 
photography. 

If  the  primary  spark  ceases  to  be  unidirectional  and 
is  allowed  to  oscillate,  the  oscillations  of  the  primary 
spark  tend  to  compel  those  of  the  secondary  or  neigh- 
bouring circuits  to  follow  them ;  if  they  are  not  suffi- 
ciently powerful  to  do  this,  they  beat  with  the  oscilla- 
tion of  the  secondary  circuit.  Moreover,  if  all  capacity 
is  removed  from  the  neighbouring  circuits,  they  oscillate 
in  tune  with  the  primary  circuit,  following  the  latter 
exactly.  The  secondary  circuits  without  capacity  act 
like  sensitive  plates  and  exactly  reproduce  every  dis- 
turbance in  the  primary  oscillating  circuit. 

In  Fig.  B,  frontispiece,  S'  represents  photographs  of 
the  unidirectional  primary  spark.  S  is  the  unidirec- 
tional spark  produced  in  a  neighbouring  circuit,  B,  from 
which  the  capacity  has  been  removed.  S"  is  the  oscil- 
lating spark  in  the  circuit  C ;  the  condenser  of  this  cir- 
cuit was  an  air  condenser.  The  spark  S  shows  that  no 
oscillation  is  concealed  by  the  heavy  pilot  spark  of  the 
exciting  spark  S'.  The  photographs  S*  show  that  the 
unidirectional  spark  S'  can  set  the  circuit  C  into  oscilla- 
tory movement,  and  that  this  oscillatory  movement  con- 


246  WHAT  IS  ELECTRICITY! 

tinues  long  after  the  exciting  blow  has  ceased.  A  care- 
ful study  of  many  photographs  of  this  nature  shows 
that  a  circuit  containing  capacity  and  self-induction  acts 
at  the  first  instant  as  if  no  capacity  were  in  the  circuit. 
It  then  begins  to  oscillate  with  a  higher  period  than  it 
afterward  reaches,  acting  at  first  like  a  closed  organ  pipe 
and  subsequently  like  a  pipe  open  at  both  ends. 

In  Fig.  A,  frontispiece,  S'  represents  again  the  oscil- 
lating primary  circuit,  S  the  oscillating  secondary  cir- 
cuit C.  The  circuits  are  nearly  in  geometrical  reso- 
nance. Slight  beats,  however,  can  be  observed.  The 
duration  of  the  secondary  is  nearly  the  same  as  that  of 
the  primary. 

As  we  advance  in  our  study  of  the  transformations 
of  electricity  we  perceive  that  we  are  driven  off,  so  to 


Fro.  45. 

speak,  from  wires  and  conductors  into  the  ether.  The 
electrical  manifestations  refuse  to  show  themselves  on 
the  conductors,  except  on  the  extreme  outer  layers  of 
such  conductors,  while  their  most  vigorous  effects  are 
displayed  in  the  ether  of  space.  We  have,  more- 
over, directed  most  of  our  attention  to  the  effects  at  the 
terminals  or  ends  of  conductors.  Let  us  now  see  if  we 
can  detect  any  form  of  wave  motion  along  the  wires 
or  conductors.  Eeturning  to  the  use  of  a  Ruhmkorff  coil 
or  step-up  transformer,  let  us  arrange  two  large  plate 
condensers,  a  and  &,  parallel  to  each  other  (Fig.  4:5)  and 


ELECTRIC  WAVES. 


247 


connect  them  with  the  terminals  of  the  step-up  trans- 
former, providing  a  spark  gap  between  B  and  D  ;  then 
place  two  other  smaller  plates,  c  and  <?,  opposite  the 
plates  a  and  b,  and  run  long  wires  from  these  smaller 
plates  fifty  or  sixty  feet  away  to  a  spark  gap  at  J. 
When  a  spark  jumps  across  the  gap  between  B  and  D 
a  spark  will  also  jump  at  J.  This  latter  spark  is  due 
to  the  rate  of  change  of  the  electrostatic  lines  between 
a  and  5.  Now,  on  walking  near  the  wires  E  F  and  ap- 
plying the  ear  very  close  to  them,  but  not  touching  them, 
one  can  find  a  point  where  a  peculiar  crackling  sound 
is  loudest ;  from  this  point  the 
sound  fades  away  in  both  di- 
rections. We  have  evidently 
detected  a  wave  of  electricity 
on  the  wires. 

Let  us  now  see  if  we  can 
make  it  evident  to  the  eyes  in- 
stead of  the  ears.  Taking  a 
glass  tube  which  has  been  rare- 
fied to  a  great  degree,  let  us 
rest  it  on  the  two  parallel  wires 
and  move  it  along  them.  When 
it  rests  at  the  place  where 
our  ears  detected  the  greatest 
sound,  the  tube  lights  up ;  the 
molecules  in  it  are  set  into 
rapid  movement.  As  we  move 
the  tube  along  the  wires  we 
'  find  that  the  brilliancy  of  its 

lighting  up  diminishes  as  we  go  in  either  direction.  We 
have  evidently  made  manifest  to  the  eyes  an  electrical 
wave.  Let  us  return  to  acoustical  analogies.  If  we 
should  connect  a  silk  thread  to  one  prong  of  a  tuning 


FIG.  46. 


248  WHAT  IS  ELECTRICITY  I 

fork  (Fig.  46),  and  support  the  other  end  on  a  cylinder 
at  A,  and  suitably  weight  the  string,  it  will  vibrate  with 
the  fork.  It  is  necessary  that  the  time  of  vibration  of 
the  fork  should  be  arranged  with  reference  to  the  length 
of  the  string ;  in  other  words,  the  string  must  be  tuned  to 
the  fork.  Now,  if  we  should  touch  the  string  at  a  node 
B  we  do  not  disturb  the  wave  form  on  the  string.  If, 
however,  we  should  touch  even  by  a  feather  the  vibrating 
portions  of  the  string,  we  will  say  at  M,  the  wave  form 
can  not  be  re-established ;  it  is  broken  up.  In  the  same 
manner,  by  suitably  lengthening  or  shortening  the  wires 
E  F  (Fig.  45),  we  can  find  places  where  a  conducting 
wire  can  bridge  the  two  wires  and  not  impair  the  bril- 
liancy of  the  light  in  the  exhausted  tube.  This  con- 
ducting wire  is  then  placed  at  the  electrical  nodes.  In 
examining  the  conditions  of  obtaining  electrical  waves, 
we  find  that  there  are  two  principal  conditions  to  be  ob- 
served :  the  number  of  lines  of  force  or  magnetic  rip- 
ples which  emanate  in  rapidly  expanding  circles  from 
every  unit  of  length  of  the  wires  E  F,  and  which  are 
thus  thrust  into  the  space  between  the  parallel  wires  E 
and  F,  and  also  the  number  of  lines  of  electrostatic 
force  between  a  and  I  and  c  and  d,  not  to  speak  of  the 
electrostatic  lines  which  extend  from  every  unit  of 
length  of  each  of  the  parallel  wires.  We  find  by  fur- 
ther study  that  the  time  of  vibration  of  the  spark  at  J, 
or,  in  other  words,  the  time  of  electrical  surging  along 
the  wires,  is  proportioned  to  the  square  root  of  the 
product  of  the  magnetic  lines  and  the  electrostatic  lines 
per  unit  length.  The  spark  at  B  D  can  be  said  to  be 
the  tuning  fork  which  maintains  the  waves  along  the 
wires.  The  spark  can  be  likened  to  a  tuning  fork  in 
resonance  with  the  fork  at  B  D,  If  we  could  now  pho- 
tograph the  spark  at  J  by  means  of  a  rapidly  revolving 


ELECTRIC  WAVES.  249 

mirror,  and  thus  spread  out  its  vibrations  so  that  they 
could  be  measured,  we  could  obtain  the  time  of  oscilla- 
tion of  the  electrical  waves  along  the  wires  E  F  ;  if  at 
the  same  time  we  measure  the  distance  between  the 
electrical  nodes  we  should  get  half  the  wave  length. 
Thus  obtaining  the  wave  length,  we  could  obtain  the 
velocity  of  propagation  of  the  electricity  along  the 
wires.  For  the  distance,  I,  which  we  call  a  wave  length, 
is  traversed  with  the  velocity,  v,  of  electricity  in  the 
time,  tf,  or,  to  express  this  by  an  equation,  I  =  v  t.  Thus 
measuring  I  and  t  we  can  obtain  v.  In  a  subsequent 
chapter  I  shall  show  how  this  has  been  accomplished, 
and  how  a  number  has  been  obtained  for  v  which  is  very 
close  to  that  of  light — about  186,000  miles  per  second. 

In  the  experiment  we  have  described  the  wave  mo- 
tion of  electricity  has  been  apparently  confined  to  wires 
or  conductors.  The  question  naturally  arises,  Can  this 
wave  motion  be  transmitted  through  space  without 
wires  ?  Hertz  has  shown  how  to  detect  these  electric 
waves  in  the  air  by  means  of  a  circle  of  wire,  the  ends 
of  which  terminate  in  a  micrometer  screw  (Fig  47),  by 
means  of  which  one  can  measure  a  very  small  spark 
gap.  The  dimensions  of  this  circle  are  so  chosen  that 
its  time  of  electrical  vibration  is  the  same  as  that  of  the 
circuit  containing  the  exciting  spark. 

On  moving  along  a  straight  horizontal  line  extend- 
ing from  the  middle  point,  M,  between  the  spark  gap  of 
the  exciter  (Fig.  47),  a  spark  will  appear  at  the  spark 
gap,  IS",  of  the  resonating  circle.  If  now  the  resonating 
circle  be  moved  to  and  fro  between  the  spark  gap  M 
and  a  large  metallic  plane  surface,  S,  certain  nodal 
points  can  be  found  at  which  the  spark  in  the  resonator 
disappears.  We  have  to  do  with  electrical  waves  which 
emanate  from  the  exciter,  strike  the  metallic  plane  sur- 


250 


WHAT  IS  ELECTRICITY! 


face,  and  are  reflected  by  the  surface.  The  phenome- 
non is  similar  to  that  we  should  obtain  if,  having  ex- 
cited a  powerful  tuning  fork  some  feet  from  a  smooth 


FIG.  47. 

wall,  we  should  obtain  evidences  of  nodal  points  be- 
tween the  prongs  of  the  fork  and  the  wall.  A  simple 
way  to  do  this  is  to  walk  toward  the  wall  with  the  fork 
while  it  is  sounding  and  note  that  there  is  a  point  where 
the  sound  of  the  fork  becomes  louder.  This  is  where 
the  reflected  wave  of  sound  re-enforces  the  movement  of 
the  prong  of  the  fork.  The  column  of  air  between  the 
fork  and  the  wall  then  vibrates  hi  time  with  the  prong 
of  the  fork,  just  as  in  the  experiment  when  the  fork  is 
held  over  a  cylinder  which  is  moved  up  and  down  in 
water  until  a  resonating  column  of  air  is  obtained. 
Sarasin  and  De  la  Eive,  working  in  a  large  room 
at  Geneva,  obtained  reflected  electric  waves  by  the 
use  of  the  circular  resonator  which  we  have  described. 
The  wave  motion  of  electricity  has  thus  been  traced 


ELECTRIC  WAVES.  251 

in  the  air,  or  rather  in  the  ether,  free  from  all  con- 
ductors. 

A  still  more  striking  way  of  showing  that  electrical 
waves  can  be  reflected  is  due  to  Hertz,  who  used  para- 
bolic mirrors.  Before  speaking  of  the  use  of  parabolic 
mirrors  to  transmit  and  receive  electrical  vibrations, 
let  us  examine  their  use  in  the  subjects  sound,  heat, 
and  light.  If  a  watch  is  held  at  the  focus  of  a  para- 
bolic mirror,  A  (Fig.  48),  and  a  listener  should  station 
himself  at  the  focus,  B,  of  a  similar  mirror  at  a  con- 
siderable distance,  he  can  hear  the  tick  of  the  watch.  If 
a  more  powerful  source  of  sound  were  placed  at  A,  a 
sound  inaudible  at  the  distance  of  forty  feet  can  be 
heard  distinctly  by  the  aid  of  the  second  mirror.  The 
sound  waves  emanating  from  the  focus  A  converge  to 
the  focus  B  of  the  receiving  mirror.  The  light  of  a 
candle  placed  at  A  will  also  be  reflected  to  the  focus 
B ;  and  the  heat  of  a  metallic  ball  placed  at  A  can  be 


FIG.  48. 

detected  when  the  focus  B  is  at  least  one  hundred  feet 
from  A.  It  only  remains  to  see  if  electrical  vibrations 
emanating  from  the  focus  A  can  be  detected  also  at 
B.  Hertz  has  shown  that  this  is  possible.  A  simple 
method  of  constructing  his  mirrors  is  the  following :  A 
framework  of  wood  is  made,  and  parabolic  curves,  cut 


252  WHAT  IS  ELECTRICITY? 

out  of  wood,  are  nailed  to  this  framework.  Over 
these  parabolic  guides  stiff  cardboard  is  nailed  and  is 
then  covered  with  tin  foil.  In  this  manner  one  can 
make  large  mirrors  of  which  every  horizontal  section  is 
a  parabola.  With  this  species  of  parabolic  mirror  we 
have  a  linear  focus  instead  of  a  focus  at  a  point.  If 
now  an  oscillatory  spark  is  formed  at  the  focus,  A,  of 
one  mirror,  electrical  waves  are  sent  out  which  are  re- 
flected from  the  surface  of  the  mirror  to  the  surface  of 
the  second  parabolic  mirror,  and  these  converge  to  the 
linear  focus  B.  If,  then,  a  suitable  conductor,  includ- 
ing a  spark  gap,  is  placed  along  this  linear  focus,  the 
waves  decay  along  this  conductor,  and  a  spark  is  ob- 
tained as  an  evidence  of  this  decay  at  the  spark  gap 
B.  When  B  was  placed  forty  feet  from  A,  sparks 
could  still  be  detected  at  B.  It  is  probable  that  this 
distance  can  be  greatly  exceeded,  and  the  imagination 
immediately  pictures  the  possibility  of  sending  and  re- 
ceiving electrical  waves  through  a  fog  between  one 
steamship  and  another.  When  the  mirrors  are  so  far 
apart  that  no  evidence  of  a  spark  can  be  obtained  at  B, 
the  focus  of  the  receiving  mirror,  the  waves  can  still  be 
detected  by  an  interesting  process  of  transformation  of 
energy.  One  terminal  of  a  galvanometer  of  great  re- 
sistance is  connected  to  one  side  of  the  spark  gap  at  B, 
and  the  other  tp  its  opposite  side.  When  the  electrical 
waves  are  received  on  the  conductors  at  B  a  sufficient 
disturbance  of  the  electrical  state  is  caused  to  produce 
a  slight  electrical  current  through  the  galvanometer. 
Prof.  Lodge  employed  another  device,  somewhat  simi- 
lar, to  detect  electrical  waves.  A  glass  tube  is  filled 
with  metallic  filings  and  is  connected  through  a  gal- 
vanometer with  a  battery.  The  inclosed  air  prevents 
electrical  contacts.  When  electrical  waves  fall  on  the 


ELECTRIC  WAVES.  253 

tube  they  cause  minute  sparks  among  the  filings,  and 
the  battery  is  connected  by  these  sparks  with  the  gal- 
vanometer. He  calls  such  tubes  coherent  tubes.  The 
sparks  serve  the  purpose  of  a  relay  to  throw  a  stronger 
electrical  impulse  through  the  galvanometer. 

Greater  refinements  can  doubtless  be  made  in  appa- 
ratus to  detect  electrical  waves.  Indeed,  our  present 
form  of  apparatus  will  doubtless  appear  to  the  worker 
fifty  years  from  now  much  as  the  rude  mirrors  of  the 
ancients  now  appear  to  us.  We  have  now  given  evidence 
that  electrical  waves  can  be  reflected  like  light  and  heat 
waves ;  it  remains  to  see  if  they  can  be  refracted  also. 
This  refraction  has  been  accomplished  by  Hertz,  who 
placed  a  large  prism  of  pitch  between  the  parabolic 
mirrors,  and  found  that,  in  order  to  obtain  evidences  of 
electrical  waves  at  the  focus  of  the  receiving  mirror,  it 
had  to  be  moved  in  order  to  receive  the  waves.  "We 
owe  to  Prof.  Rhigi,  of  Bologna,  a  great  simplification 
of  Hertz's  apparatus,  and  it  is  interesting  to  reflect  that 
by  means  of  this  simplification  we  are  brought  back  to 
the  use  of  the  electrical  machine.  We  are  again  in  the 
position  of  Benjamin  Franklin  in  respect  to  apparatus. 
With  great  experience  gained  by  means  of  the  re- 
searches of  Galvani,  Yolta,  and  Faraday,  we  have  been 
led  back  by  a  distinguished  Italian  to  the  study  of  elec- 
tricity in  its  most  untrammelled  manifestation.  In- 
stead of  a  Kuhmkorff  coil  or  a  transformer  excited  by 
a  battery,  Rhigi  uses  a  small  electrical  machine  such  as 
we  have  shown  (Fig.  2,  page  27)  in  comparison  with  a 
Franklin  machine.  Two  little  spheres  in  oil  (B  and  C, 
Fig.  49)  receive  their  charges  from  the  prime  conduct- 
ors, P  and  P',  of  the  electrical  machine.  Three  sparks 
are  formed,  but  the  middle  spark,  B  C,  is  the  oscilla- 
tory one  of  very  rapid  period ;  for  the  capacity  of  B 


254:  WHAT  IS  ELECTRICITY? 

and  C  is  very  small,  and  so  also  is  the  self-induction  of 
the  little  portion  of  the  circuit,  A  B  C  D.  The  di- 
mensions of  the  spheres  and  the  circuit  can  be  made 
so  small  *  that  electric  waves  of  six  millimetres  (about 


FIG.  49. 

one  quarter  of  an  inch)  can  be  sent  out  from  this  oscil- 
lator, whereas  the  shortest  waves  obtained  by  Hertz 
were  several  feet  long.  In  order  to  detect  these  waves, 
it  was  necessary  also  to  have  a  resonator  or  electrical 
eye  of  very  small  capacity  and  self-induction.  Bhigi 
accomplished  this  by  coating  a  plate  of  glass  with  tin 
foil  and  making  with  a  diamond  a  thin  cut  in  the  tin 
foil.  Minute  sparks  passed  across  this  cut  when  the 
detector  was  in  electrical  tune  with  the  little  spheres  in 
the  oil. 

With  this  apparatus,  experiments  on  electrical  waves 
are  brought  within  the  range  of  almost  any  experi- 
menter, and  with  it  Khigi  has  performed  by  means  of 
electrical  waves  almost  all  the  ordinary  optical  experi- 
ments, such  as  refraction  of  waves  by  prisms  of  pitch 
or  other  light  opaque  insulating  substances ;  interfer- 
ence of  waves ;  reflection  of  waves,  from  the  focus  of 
one  cylindrical  or  spherical  mirror  to  the  focus  of  an- 
other ;  and  polarization  of  electric  waves. 

The  experiment  of  polarization  has  been  shown  by 
a  block  of  wood  in  the  following  manner :  In  Fig.  50 

*  Lebedew,  Ann.  der  Physik.  und  Chemic,  vol.  Ivi,  1895. 


ELECTRIC  WAVES. 


255 


are  represented  the  two  parabolic  mirrors  used  by 
Hertz  to  show  reflection  of  electro-magnetic  waves,  A 
being  the  transmitter  and  B  the  receiver.  B  is  turned 
so  that  its  focal  line  is  at  right  angles  to  that  of  A.  In 
this  position  no  sparks  are  observable  in  the  little  res- 
onator in  the  focal  line  of  B.  When  a  block  of  wood, 
however,  is  placed  in  the  line  between  the  exciter  or 
oscillator  of  A  and  the  resonator  of  B,  in  certain  posi- 
tions of  the  grain  of  the  wood  sparks  begin  to  appear 
in  the  resonator.  The  wood  allows  the  waves  to  pass 
through  in  certain  positions  of  its  fibres,  and  shuts  them 
out  in  other  positions.  In  other  words,  it  polarizes  the 
waves,  just  as  certain  substances  of  different  molecular 
aggregations  in  different  directions  polarize  light  waves. 
Lebedew  has  used  little  spherical  mirrors  instead  of 
parabolic  mirrors,  and  has  reduced  the  dimensions  of 
the  apparatus  to  almost  the  size  of  optical  apparatus  for 
measuring  wave  lengths  of  light.  Instead  of  the  res- 


FIG.  50. 

onator  of  Khigi,  he  employs  for  detector  of  waves  in 
the  air  a  thermal  junction.  Thus  we  have  now  three 
forms  of  electrical  eye,  so  to  speak :  the  micrometer 
spark  gap  used  by  Hertz  (Fig.  51,  M) ;  the  plate  of 
glass  with  the  extremely  fine  diamond  cut  across  its 
tin-foil-coated  surface  N;  and  the  thermal  junction, 
first  used  by  Klemencic  O.  With  the  spark  gap  we 


256 


WHAT  IS  ELECTRICITY! 


detect  the  waves  by  the  light  of  the  sparks ;  with  the 
thermal  junction  we  detect  the  waves  surging  between 
A  and  B  by  the  heat  developed  in  the  very  fine  junc- 
tion J.  On  account  of 
the  self-induction,  or 
electrical  inertia  of  the 
galvanometer  circuit, 
the  waves  do  not  pass 
along  this,  and  their  os- 
cillations are  confined 
between  A  and  B. 

The  electro-magnet- 
ic waves  pass  through 
brick  walls  unaffected 
by  them.  They  are 
probably  not  absorbed 
by  fog,  and  therefore  if 
we  could  detect  such 
waves  at  a  distance  of  a 
thousand  feet  we  should 
have  a  method  of  sig- 
nalling through  a  dense 
fog,  and  possibly  a  method  of  preventing  collisions  at 
sea.  The  most  powerful  electric  light  can  not  penetrate 
a  dense  fog  this  distance.  Unfortunately,  at  present  we 
can  not  detect  the  electro-magnetic  waves  more  than 
one  hundred  feet  from  their  source.  To  do  this  we  are 
obliged  to  employ  the  large  parabolic  mirrors  of  Hertz, 
and  a  comparatively  small  spark  at  the  focus  of  one 
mirror  and  a  Ehigi  detector  at  the  focus  of  the  receiving 
mirror.  Yery  powerful  sparks  do  not  seem  to  work  as 
well  as  comparatively  feeble  ones.  I  have  had  con- 
structed parabolic  mirrors  nearly  six  feet  high  and  six 
feet  across.  Such  mirrors  are  twice  the  size  of  those 


FIG.  61. 


ELECTRIC  WAVES.  257 

employed  by  Hertz,  but  I  have  not  hitherto  been  able  to 
greatly  extend  the  distance  at  which  the  electrical  waves 
become  too  feeble  to  be  distinguished.  Lord  Rayleigh 
has  shown  that  with  long  waves  of  sound  a  plane  sur- 
face is  as  good  a  transmitter  or  reflector  as  a  curved 
surface.  It  is  probable,  also,  that  with  very  powerful 
sparks  a  plane  mirror  would  answer  better  than  a  para- 
bolic mirror  or  a  spherical  mirror,  for  it  is  impossible 
to  produce  a  very  powerful  electric  spark  with  a  very 
rapid  rate  of  oscillation,  on  account  of  the  large  amount 
of  capacity  necessary.  The  waves,  therefore,  must  be 
several  feet  in  length.  "With  a  flat  mirror  we  should 
avoid  the  diffraction  or  bending  of  the  wave  about  the 
edges  of  the  curved  mirror.  The  principal  difficulty, 
however,  of  extending  the  range  of  electro-magnetic 
waves  is  in  their  rapid  rate  of  decay,  or  damping,  as  it 
is  termed.  The  light  of  the  electric  spark  can  be  de- 
tected by  the  eye  much  farther  than  the  electro-mag- 
netic waves  it  sends  forth.  Yet  this  light  is  caused  by 
the  electric  waves.  The  electro-magnetic  waves  have 
decreased  so  greatly  in  amplitude  when  they  have  reached 
the  position  of  the  observer's  eye  that  they  have  not 
sufficient  energy  to  start  waves  in  any  detector  with 
which  we  are  now  acquainted.  They  can,  however,  dis- 
turb the  ether  so  as  to  give  the  sensation  of  light. 

A  very  important  criticism  on  the  present  methods 
of  measuring  the  reflection  of  electric  waves  and  their 
supposed  interferences  in  air  has  been  made  by  Bjerk- 
nes.  He  points  out  that  the  amplitude  or  height  of 
the  electric  waves  change  as  they  progress  away  from 
their  source,  while  the  amplitude  of  plane  light  waves 
remain  unchanged.  The  electric  waves  can  be  repre- 
sented by  curve  similar  to  the  curves  in  Fig.  52,  in 
which  the  successive  crests  diminish  or  increase  in 


258 


WHAT  IS  ELECTRICITY? 


height,  due  to  conditions  of  the  oscillating  source  of 
electricity,  while  the  regular  amplitudes  of  the  light 
waves  can  be  represented  by  the  regularly  undulating 
curve  of  Fig.  15,  page  143.  Furthermore,  we  study 
optical  phenomena  by  apparatus  which  is  inert,  so  to 
speak — that  is,  the  apparatus  does  not  produce  waves  of 
light  itself,  which  complicate  the  observations.  Thus 
we  observe  the  spectrum  by  means  of  a  telescope ;  we 

study  the  inter- 
ference bands  of 
light  produced, 
for  instance,  by 
light  in  passing 
through  a  very 
narrow  slit  by 
means  of  the  eye. 
Now  the  tele- 
scope and  the  eye 
do  not  produce 
waves  of  light 
which  interfere 
with  the  obser- 
vations. In  the 
study,  however, 
of  electro -mag- 
netic waves  in  air  our  only  method  at  present  is  by  the 
employment  of  electric  circuits  in  which  a  spark  gap  is 
placed.  Now  the  electric  wave  sent  out  from  the  spark 
gap  of  the  exploring  circuit  also  has  a  varying  ampli- 
tude, and  therefore  when  we  move,  for  instance,  our  res- 
onator— that  is,  the  little  circuit  inclosing  a  spark  gap — 
to  and  fro  in  front  of  a  wall  which  is  reflecting  electric 
waves,  the  interferences  we  perceive  may  be  merely 
the  interferences  between  the  decreasing  amplitudes  of 


FIG.  52. 


ELECTRIC  WAVES.  259 

the  wave  reflected  from  the  wall  and  the  changing 
amplitude  of  the  wave  in  the  resonator.* 

If  we  look  at  the  incandescent  filament  of  a  distant 
Edison  lamp  through  a  narrow  slit  cut  in  card,  or  if 
we  gaze  at  it  along  the  edge  of  the  card,  we  perceive 
interference  bands  of  light,  consisting  of  bright  and 
dark  spaces.  The  eye  may  be  called  an  indifferent  in- 
strument ;  it  passively  accepts  the  phenomena.  The 
waves  of  light  interfere  with  each  other  on  the  retina. 
When  the  crest  of  one  wave  coincides  with  the  trough 
of  another,  no  impression  is  made  on  the  retina ;  we  see 
a  dark  space.  When  the  crests  of  the  waves  coincide,  we 
are  conscious  of  a  bright  space.  If  now  the  retina  oscil- 
lated, so  to  speak,  or  if,  on  being  excited,  it  sent  out  waves 
also,  it  is  evident  that  we  should  be  conscious  of  very 
complicated  phenomena  of  interference.  The  waves  in 
our  eyes  might  annul  tlje  waves  coming  from  the  outer 
object  so  that  we  might  not  be  conscious  of  the  outer 
light. 

The  experiments  on  electrical  resonance  lead  us  to 
believe  strongly  in  the  existence  of  a  periodic  electrical 
disturbance  in  some  medium  filling  space,  which  is 
propagated  with  the  velocity  of  light.  The  velocity  of 
electricity  in  this  medium,  according  to  Maxwell,  is  the 
same  as  the  velocity  of  light.  This  is  evidently  a  very 
important  point  to  establish  in  regard  to  Maxwell's 
great  generalization,  and  in  connection  with  Mr.  Duane, 
one  of  my  graduate  students,  I  have  carefully  measured 
the  velocity  of  electric  waves  by  the  following  method, 
which  is  free  from  estimations  of  electrical  dimensions 
of  the  circuit,  and  by  which  the  wave  lengths  and  the 
time  were  measured  directly  on  the  same  circuit  along 

*V.  Bjerknes,  Annalen  Physik  und  Chemie,  No.  1,  1895. 
IS 


260  WHAT  IS  ELECTRICITY? 

which  the  wave  motion  was  propagated.  This  method 
of  procedure  is  of  great  importance  ;  for  we  have  seen 
that  the  early  methods  of  measurement  of  the  velocity 
of  electricity  were  vitiated,  so  to  speak,  by  the  conditions 
of  the  apparatus  which  was  used  for  the  measurement, 
and  later  calculations  depend  upon  estimations  of  self- 
induction  and  of  the  time  of  the  periodic  motion  of  a 
circuit,  which  is  not  the  same  as  that  along  which  the 
motion  takes  place.  In  short,  our  measure  is  a  direct 
measure  of  the  speed  of  the  electric  waves  in  the  ether 
near  the  surface  of  the  wires  we  employed. 

The  arrangement  and  dimensions  of  the  apparatus 
finally  adopted  were  as  follows  (see  Fig.  45,  page  246) : 

Two  metallic  plates,  a  and  Z>,  30  X  30  centimetres, 
placed  in  vertical  planes,  formed  the  primary  condens- 
er. The  dielectric  between  them  consisted  of  the  best 
French  plate  glass  obtainable,  and  was  two  centimetres 
thick.  Outside  the  plates  a  and  5,  and  separated  from 
them  by  a  hard-rubber  dielectric,  1-8  centimetre  thick 
were  the  secondary  plates  26  X  26  centimetres.  The 
primary  and  secondary  circuits  were  joined  to  the  con- 
denser plates  as  indicated  in  the  iigure.  The  primary 
circuit  lay  in  the  horizontal  plane  passing  through  the 
centres  of  condenser  plates,  and  consisted  of  copper 
wires,  0'34  centimetre  in  diameter.  In  order  to  control 
the  period  of  oscillation  of  the  primary  circuit,  the  por- 
tion B  D,  containing  a  spark  gap  with  spherical  ter- 
minals, was  made  to  slide  along  parallel  to  itself.  The 
distance  between  the  straight  portions  A  B  and  C  D 
was  40  centimetres,  and  the  lengths  of  A  B  and  C  D 
finally  chosen  for  best  resonance  were  85  centimetres. 
Most  of  the  secondary  circuit  lay  in  a  horizontal  plane 
15  centimetres  above  that  of  the  primary.  The  lengths 
GE  and  HF,  however,  were  bent  down  and  fastened 


ELECTRIC   WAVES.  261 

to  the  middle  points,  G  and  H,  of  the  secondary  plates. 
The  circuit  consisted  of  copper  wire  (diameter  0*215 
centimetre),  and  its  total  length  from  G  through  I  to 
H  was  5,860  centimetres.  At  I  was  a  spark  gap  with 
pointed  terminals.  With  this  apparatus  we  succeeded 
in  producing  a  very  regular  wave  formation,  as  indi- 
cated by  the  bolometer.  There  was  a  node  at  I,  and 
another  about  40  centimetres  to  the  right  of  E  and  F. 

The  images  of  the  secondary  spark  were  thrown  on 
a  sensitive  plate  by  means  of  a  rotating  mirror.  The 
dots  obtained  represent  discharges  from  the  negative 
terminals  only,  the  positive  discharges  not  being  brilliant 
enough  to  affect  the  plate.  The  distance  between  suc- 
cessive dots  was  the  distance  on  the  plate  through  which 
the  image  of  the  spark  gap  moved  during  the  time  of  a 
complete  oscillation.  Hence  by  determining  the  speed 
of  the  mirror  and  measuring  the  distances  from  the 
mirror  to  the  plate  the  time  of  oscillation  could  be 
calculated.  To  measure  the  sparks,  we  used  a  sharp 
pointer  moved  at  the  end  of  a  micrometer  screw,  under 
a  magnifying  glass  of  low  power.  The  instrument  was 
originally  intended  for  microscopic  measurements,  and 
was  very  accurately  constructed.  The  rotating  mirror 
was  driven  by  an  electric  motor  by  means  of  a  current 
from  a  storage  battery  of  extremely  constant  voltage. 
To  give  great  steadiness,  a  heavy  fly  wheel  was  attached 
to  the  axis  of  the  mirror.  The  speed  of  the  mirror  was 
determined  to  within  about  one  part  in  five  hundred  by 
means  of  an  electric  chronograph. 

It  appears  from  the  best  results  that  we  have  ob- 
tained that  the  velocity  of  short  electric  waves  travel- 
ling along  two  parallel  wires  differs  from  the  velocity 
of  light  by  less  than  two  per  cent  of  its  value.  It  has 
been  shown  theoretically  that  the  velocity  of  such 


262  WHAT  IS  ELECTRICITY! 

waves  travelling  along  a  single  wire  should  be  the  ve- 
locity of  light  approximately.  These  results,  therefore, 
in  a  certain  sense,  confirm  the  theory,  to  an  accuracy 
within  their  probable  error. 

Theoretically,  too,  the  velocity  should  be  approxi- 
mately equal  to  the  ratio  between  the  two  systems  of 
electrical  units.  The  average  of  the  best  measure- 
ments of  this  ratio  is  3'001,  which  is  nearer  the  average 
velocity  obtained  for  electric  waves  than  the  velocity 
of  light. 

We  have  established,  I  believe,  beyond  reasonable 
doubt,  that  the  waves  of  electricity  travel  with  the  ve- 
locity of  light,  for  the  waves  on  the  wires  we  employed 
must  have  been  proved  to  reside  entirely  on  the  surface 
— that  is,  on  the  boundary,  so  to  speak,  of  the  medium 
pervading  the  space  about  the  wires.  The  formation  of 
stationary  electric  waves  which  are  propagated  with  the 
velocity  of  light  is  the  best  evidence  we  now  have  of 
the  truth  of  Maxwell's  great  generalization. 

We  have  said  that  Joseph  Henry  showed  at  an  early 
date  that  the  discharge  of  a  Ley  den  jar  is  oscillatory. 
Our  present  knowledge  of  electric  waves  is  largely  due 
to  a  realizing  sense  of  the  importance  of  the  observa- 
tions of  Henry.  One  will  find  in  his  published  paper 
the  following  remarkable  conclusion,  which  can  be 
regarded  almost  as  a  prophecy : 

"  In  extending  the  researches  relative  to  this  part  of 
the  investigations,  a  remarkable  result  was  obtained  in 
regard  to  the  distance  at  which  induction  effects  are 
produced  by  a  very  small  quantity  of  electricity.  A 
single  spark  from  the  prime  conductor  of  a  machine  of 
about  an  inch  long,  thrown  on  to  the  end  of  a  circuit 
of  wire  in  an  upper  room,  produced  an  induction  suffi- 
ciently powerful  to  magnetize  needles  in  a  parallel  cir- 


ELECTRIC  WAVES.  263 

cuit  of  iron  placed  in  the  cellar  beneath,  at  a  perpen- 
dicular distance  of  30  feet,  with  two  floors  and  ceilings, 
each  14  inches  thick,  intervening.  The  author  is  dis- 
posed to  adopt  the  hypothesis  of  an  electrical  plenum, 
and  from  the  foregoing  experiments  it  would  appear 
that  a  single  spark  is  sufficient  to  disturb  perceptibly 
the  electricity  of  space  throughout  at  least  a  cube  of 
400,000  feet  of  capacity.  And  when  it  is  considered 
that  the  magnetism  of  the  needle  is  the  result  of  the 
difference  of  two  actions,  it  may  be  further  inferred 
that  the  diffusion  of  motion  in  this  case  is  almost  com- 
parable with  that  of  a  spark  from  a  flint  and  steel  in 
the  case  of  light."  * 

*  Scientific  Writings  of  Joseph  Henry,  vol.  i,  p.  202,  Smithsonian 
Institution,  Washington. 


CHAPTEE  XIX. 

THE   ELECTRO-MAGNETIC   THEORY   OF    LIGHT   AND 
THE   ETHEK. 

THE  various  phenomena  of  the  action  between  mag- 
nets— the  induction  phenomena  between  neighbour- 
ing circuits,  a  current  of  induction  rising  in  one  circuit 
whenever  an  electric  current  is  started  in  a  neigh- 
bouring circuit,  and  thus  manifesting  energy — lead  us 
to  believe  that  the  energy  has  been  transferred  from 
the  exciting  circuit  across  space  by  means  of  some  me- 
dium filling  that  space.  The  energy  has  disappeared 
from  the  exciting  circuit,  and  has  reappeared  in  the  in- 
duction circuit.  It  must  have  existed  during  the  time 
of  its  disappearance  and  reappearance  in  the  interven- 
ing space.  We  are  therefore  forced  to  believe  in  some 
medium  which  serves  to  convey  this  energy. 

The  old  fluid  theories  implied  that  when  a  body 
was  electrified  it  had  something  upon  it  which  was 
called  electricity.  According  to  the  modern  views,  we 
regard  the  ether  around  the  body  as  charged  with  en- 
ergy which  is  the  result  of  the  work  we  have  done  in 
charging  the  body.  This  energy  in  the  ether  is  the 
energy  of  motion.  There  is  a  state  of  strain  in  the 
ether  which  we  term  a  polarized  condition.  Around  a 
positively  charged  body  this  polarization  has  a  certain 
direction  and  a  certain  amount.  With  a  negatively 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.  265 

charged  body  this  polarization  is  in  an  opposite  direction. 
It  is  suggested  that  these  polarizations  may  be  like 
right-handed  and  left-handed  rotations  or  twists. 
When  we  electrify  a  conductor  we  store  up  energy 
around  the  conductor  in  the  ether.  The  work  we  do 
is  spent  in  changing  the  state  of  the  medium.  When 
a  body  is  discharged,  the  medium  returns  to  its  original 
state,  and  the  energy  is  dissipated  as  heat  in  the  electric 
spark  or  as  heat  in  the  conductor.  The  electric  current 
is  therefore  the  manifestation  of  energy  in  the  ether 
along  the  wire  through  which  the  current  appears  to 
flow.  According  to  Poynting's  theory,  we  have  seen 
that  the  electric  energy  produced  by  a  battery  or  a 
dynamo  does  not  flow  along  the  wire — for  instance, 
the  overhead  wire  of  an  electric  railroad — but  it  pro- 
duces this  strained  condition  in  the  ether,  and  the  ether 
relieves  itself  along  the  wire.  What  we  call  the  flow 
of  the  electrical  current  is  therefore  not  in  the  same 
direction  as  the  flow  of  electrical  energy. 

When  a  Leyden  jar  is  discharged  the  knobs  of  the 
jar  become  alternately  positive  and  negative.  The  me- 
dium around  the  jar  is  therefore  polarized  alternately  in 
opposite  directions.  This  polarization  starts  from  the 
knob  and  spreads  through  space,  at  each  point  of  which 
there  are  to-and-fro  motions,  and  waves  of  opposite  po- 
larizations are  sent  through  the  medium,  carrying  the 
energy  which  had  been  stored  up  in  the  Leyden  jar. 
There  is  a  periodic  or  to-and-fro  movement  in  the  ether, 
and  if  we  could  make  a  Leyden  jar  of  molecular  dimen- 
sions charge  it  and  discharge  it,  we  could  produce  a  peri- 
odic movement  in  the  ether  which  is  analogous  to  that 
which  occurs  in  the  propagation  of  light.  Maxwell's  elec- 
tro-magnetic theory  of  light  supposes  that  the  periodic 
motions  which  constitute  light  are  of  the  same  nature 


266  WHAT  IS  ELECTRICITY? 

as  those  which  arise  when  the  positive  and  negative 
conditions  of  the  ether  are  rapidly  alternated  in  the 
case  of  the  discharge  of  a  Leyden  jar.  Light,  heat, 
and  electricity  are  therefore  manifestations  of  electro- 
magnetic waves  which  come  to  us  from  the  sun. 

In  our  study  of  electric  waves  we  have  used  their 
light  manifestations  in  order  to  trace  their  phenomena. 
Thus,  when  a  Leyden  jar  is  discharged  through  a  great 
circle  of  wire  properly  placed  in  a  room,  we  can  send 
electro-magnetic  waves  through  brick  walls  and  detect 
them  in  neighbouring  rooms  by  the  sparks  that  are  ex- 
cited in  a  similar  circle  of  wire  connected  to  another 
similar  Leyden  jar.  By  photographing  the  latter  spark, 
we  can  say,  in  popular  language,  that  we  have  photo- 
graphed by  means  of  waves  that  have  passed  through 
a  brick  wall.  We  shall  see  later  that  it  is  possible  to 
photograph  by  means  of  electric  waves  which  have 
passed  through  opaque  metallic  screens,  which  cut  off 
entirely  the  light  rays  so  considered. 

Our  eyes  can  see  an  electric  spark  very  much  farther 
than  we  can  detect  the  electro-magnetic  waves  by  any 
other  instrument  than  the  eye.  The  eye  really  detects 
them  in  the  form  of  light.  It  is  true  also  that  we  can 
not  detect  the  heat  waves  sent  out  by  the  spark  so  far  as 
we  can  perceive  the  electric  waves.  The  heat  waves  are 
nearer  in  length  to  the  electric  waves  which  we  can  de- 
tect than  the  light  waves.  The  most  delicate  thermom- 
eter will  not  show  any  indication  of  heat  at  a  distance  of 
ten  feet  from  a  powerful  spark  if  we  prevent  the  electro- 
magnetic waves  from  surging  in  its  mass,  and  if  we  de- 
pend upon  the  direct  radiation  of  heat  through  the  ether. 

We  are  thus  certainly  as  advantageously  placed  at 
present  in  regard  to  measuring  electro-magnetic  waves 
generated  by  a  spark  as  we  are  in  regard  to  measuring 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    267 

heat  waves  which  accompany  them.  We  can,  however, 
measure  the  long  waves  of  heat  which  come  to  us  from 
the  sun,  yet  we  can  not  detect  the  long  electro-mag- 
netic waves.  Prof.  J.  J.  Thomson  has  shown  that  if 
an  electrical  charge  on  a  sphere  is  disturbed  in  any 
sudden  way,  it  can  oscillate  to  and  fro  in  the  time  taken 
by  light  to  travel  a  certain  number  of  tunes  the  diame- 
ter of  the  sphere,  depending  on  the  wave  length  of  the 
electric  wave.  Prof.  O.  J.  Lodge,  in  quoting  this  cal- 
culation, remarks  :  "  An  electrostatic  charge  on  the 
whole  earth  would  surge  to  and  fro  seventeen  times  a 
second.  On  the  sun  an  electric  swing  lasts  six  and  a 
half  seconds.  Such  a  swing  as  this  would  emit  waves 
19  x  106  kilometres  or  1,200,000  miles  long,  which,  travel- 
ling with  the  velocity  of  light,  could  easily  disturb  mag- 
netic needles  and  produce  auroral  effects,  just  as  smaller 
waves  produce  sparks  in  gilt  wall  paper  (Khigi's  res- 
onators), or  as  the  still  smaller  waves  of  Hertz  pro- 
duce sparks  in  his  little  resonators,  or,  once  more,  as  the 
waves  emitted  by  electrostatically  charged  vibrating 
atoms  excite  corresponding  vibrations  in  our  retina."  * 
With  our  present  methods  of  studying  electric 
waves  we  are  compelled  to  use  electric  sparks  which  do 
not  succeed  each  other  continuously.  The  electric 
waves  from  one  spark  are  thousands  of  miles  on  their 
way  before  another  spark  occurs.  The  dying  out  of  the 
waves,  or  what  we  have  called  their  damping,  is  due  to 
the  rapid  radiation  of  the  energy  of  the  spark  into  space. 
Hertz  has  calculated  this  loss  of  energy  in  the  case  of  a 
small  spark  from  two  spheres  of  fifteen  centimetres 
radius  (about  six  inches)  so  arranged  that  they  sent  out 
waves  four  hundred  and  eighty  centimetres  long  (one 

*  Lightning  Conductors  and  Lightning  Guards,  p.  260. 


268  WHAT  IS  ELECTRICITY? 

hundred  and  ninety-two  inches).  He  estimates  that 
such  an  oscillator  giving  waves  of  this  length  must  be 
supplied  with  energy  at  the  rate  of  twenty-two  horse 
power  per  second  if  the  intensity  is  to  be  kept  constant. 
"At  a  distance  of  twelve  metres  (thirty-nine  and  a 
half  feet)  from  the  spheres  or  oscillator  the  intensity  of 
the  radiation  is  equal  to  the  intensity  of  the  solar  radi- 
ation at  the  surface  of  the  earth."  * 

From  this  calculation  we  see  what  an  enormous 
amount  of  energy  is  radiated  in  a  stroke  of  lightning. 
This  consideration  of  the  rapid  damping  of  electric 
waves  sent  out  by  electric  sparks  is  interesting  from  the 
point  of  view  of  the  endeavour  to  obtain  light  by  rapid 
electric  oscillations. 

The  hypothesis  of  an  ether  filling  all  space,  a  medium 
by  means  of  which  light,  heat,  and  electricity  are  trans- 
mitted to  the  earth  from  the  sun,  seems  untenable  to 
many  minds,  largely  because  we  are  obliged  to  attribute 
to  this  medium  extraordinary  qualities  of  rigidity  and 
elasticity,  and  to  regard  it  of  such  extreme  tenuity  that  it 
escapes  detection  by  direct  measurement ;  for  it  does 
not  impede  the  motion  of  bodies. 

If  one  critically  examines  the  advance  that  has  taken 
place  in  the  applications  of  electricity,  one  can  see  that 
even  the  practical  man's  chief  consideration  is  in  re- 
gard to  the  medium  around  his  electro-magnets  and 
his  currents.  In  designing  dynamos  and  motors  for 
the  transmission  of  power,  one  of  the  main  principles  to 
be  borne  in  mind  is  that  movable  parts  of  iron  or 
copper  will  arrange  themselves  in  a  magnetic  field  so  as 
to  diminish  as  far  as  possible  the  magnetic  resistance  of 
such  a  field.  By  magnetic  resistance  we  mean  the  dif- 

*  Preston,  Theory  of  Light,  p.  444. 


THE  ELECTRO-MAGNETIC  THEORY  OP  LIGHT.    209 

ficulty  different  media  oppose  to  the  establishment  of 
magnetic  lines  of  force.  Thus  an  iron  nail  will  set 
itself  along  the  lines  of  force,  so  that  they  may  pass 
through  it  rather  than  be  spread  through  the  medium 
outside  the  iron.  A  copper  cent  will  tend  to  present 
its  edge  to  the  magnetic  pole,  for  this  position  offers 
the  least  resistance  in  the  field  to  the  establishment  of 
the  lines  of  force  or  the  stress  in  the  medium. 

This  great  law,  that  bodies  arrange  themselves  so  as 
to  diminish  the  electrical  distance  of  the  field  in  which 
they  are  placed,  enables  us  to  detect  rapid  to-and-fro 
currents  which  our  compasses  and  galvanometers  are 
utterly  unable  to  show  us.  If,  for  instance,  we  should 
put  a  compass  near  a  telephone  through  which  we  speak, 
no  movement  will  be  perceived  of  the  needle  of  the  most 
delicate  compass ;  if,  however,  we  suspend  a  piece  of 
soft  iron  in  front  of  a  telephone  magnet,  so  that  it 
makes  a  certain  angle  with  the  axis  of  the  magnet,  and 
provide  it  with  a  tiny  mirror,  we  shall  find  that  when 
we  speak  through  the  telephonic  circuit  the  little  bar  of 
iron  will  move  so  as  to  place  itself  along  the  direction 
of  the  fluctuating  lines  of  magnetic  force.  It  thus  be- 
trays the  distance  of  to-and-fro  electric  currents  in 
electro-magnetic  coils  which  otherwise  might  be  unsus- 
pected. The  stress  does  not  reside  in  the  air,  for  the 
magnetic  lines  stream  through  the  best  vacuum  we  can 
produce.  The  stress  is  in  the  ether. 

A  similar  law  holds  in  regard  to  lines  of  electric 
force  which  exist  about  bodies  charged  with  electricity. 
If  we  should  hang  up  in  a  room  a  charged  pith  ball, 
these  electrostatic  lines  extend  from  the  pith  ball  to  the 
walls  of  the  room ;  they  indicate  a  stress  or  strain  in 
the  medium.  All  the  particles  of  dust  in  the  room  tend 
to  place  themselves  so  as  to  diminish  the  resistance  of 


270  WHAT  IS  ELECTRICITY? 

the  medium  to  the  establishment  of  these  electrostatic 
lines.  If  the  length,  for  instance,  of  a  human  hair  is 
in  the  direction  of  one  of  these  lines  of  force,  it  will 
not  indicate  any  rapid  change  in  electrification  of  the 
pith  ball ;  but  if  it  is  not  in  such  a  direction  it  will  make 
manifest  by  its  movement  what  would  not  be  otherwise 
revealed.  The  electrical  engineers  use  in  their  calcula- 
tions a  term  which  is  called  the  permeability  of  the 
medium  for  magnetic  lines  of  flow. 

The  phenomena  of  electro-magnetism,  then,  compel 
us  to  assume  the  existence  of  a  medium  through  which 
and  by  means  of  which  the  electrical  energy  is  trans- 
mitted. In  the  case  of  electro-magnetism,  we  observe 
that  certain  lines  of  force  are  established  in  the  space 
around  magnets,  and  around  wires  carrying  currents. 
While  these  lines  are  being  established,  work  must  be 
done  on  the  medium,  and  energy  is  stored  up  in  the 
medium.  When  the  lines  of  force  are  withdrawn 
from  the  medium,  the  energy  that  was  stored  up  now 
rushes  into  the  wire  of  the  circuit  of  the  electro-magnet 
and  is  dissipated  as  heat. 

The  phenomenon  of  self-induction  manifests  this 
storing  of  energy  in  the  medium ;  and  it  is  only  within 
a  few  years  comparatively  that  we  have  obtained  a  clear 
conception  of  what  we  now  call  self-induction  or  in- 
ductance. In  the  subject  of  electricity  we  may  be  said 
to  rely  upon  the  phenomenon  of  induction  to  prove  the 
necessity  of  a  medium  between  the  pith-ball  magnets 
or  electric  circuits  which  manifest  the  varied  phenom- 
ena of  induction.  Since  the  phenomena  of  light  seem 
to  require  the  existence  of  a  medium  as  well  as  the 
phenomena  of  electricity,  a  careful  search  has  been 
made  to  establish  a  relation  between  light  and  electro- 
magnetism  by  means  of  experiment.  To  establish  such 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    271 

a  relation  we  must  discover  some  phenomenon  of  light 
which  is  affected  by  phenomena  of  magnetism.  Mrs. 
Somerville  endeavoured  to  magnetize  a  needle  by  letting 
a  beam  of  light  fall  upon  it,  and  apparently  succeeded. 
Maxwell  remarks  in  regard  to  this  noted  experiment 
that  "  we  must  remember  that  the  distinction  between 
magnetic  north  and  south  is  a  mere  matter  of  direction, 
and  would  be  at  once  reversed  if  we  reverse  certain 
conventions  about  the  use  of  mathematical  signs.  There 
is  nothing  hi  magnetism  analogous  to  the  phenomena 
of  electrolysis  which  enable  us  to  distinguish  positive 
from  negative  electricity,  by  observing  that  oxygen  ap- 
pears at  one  pole  of  a  cell  and  hydrogen  at  the  other. 
Hence  we  must  not  expect  that  if  we  make  light  fall  on 
one  end  of  a  needle,  that  end  will  become  a  pole  of  a 
certain  name,  for  the  two  poles  do  not  differ  as  light 
does  from  darkness."  * 

There  are,  however,  two  experiments  which  appear 
to  establish  a  certain  experimental  relation  between 
light  phenomena  and  magnetic  phenomena.  Faraday, 
in  seeking  such  a  relation,  discovered  that  if  a  beam  of 
polarized  light  (see  p.  80)  is  sent  through  a  parallelo- 
pipedon  of  dense  glass  placed  in  the  core  of  a  powerful 
electro-magnet,  and  if  it  is  examined  by  an  analyzer 
(see  p.  80),  it  is  found  that  the  plane  of  polarization  is 
rotated  through  a  certain  angle.  One  therefore  has  to 
turn  the  analyzer  around  its  axis  to  shut  off  the  light 
which  is  now  transmitted  through  the  two  Nicols. 
The  direction  in  which  one  must  turn  the  analyzing 
Nicol  depends  upon  the  direction  of  the  current  which 
excites  the  electro-magnet.  Here  is  evidently  a  rela- 
tion between  light  and  electro-magnetism.  The  mag- 

*  Maxwell's  Electricity  and  Magnetism,  vol.  ii,  p.  410. 


272  WHAT  IS  ELECTRICITY! 

netic  force  through  the  ^lass  coincides  with  the  direc- 
tion of  the  ray  of  light ;  and  the  effect  of  this  mag- 
netic force  is  to  turn  the  plane  of  polarization  around 
the  direction  of  the  ray  as  an  axis.  This  effect  has 
been  noticed  in  a  number  of  substances  besides  glass, 
and  the  amount  of  the  turning  depends  upon  the  nature 
of  the  substance.  It  has  also  been  discovered  that  if  a 
beam  of  polarized  light  is  allowed  to  fall  on  the  surface 
of  the  iron  core  of  a  powerful  electro-magnet  its  plane 
of  polarization  is  rotated. 

After  a  careful  consideration  of  phenomena  of  this 
nature,  Maxwell  concludes  that  "  there  is  nothing  in  the 
magnetic  phenomena  which  corresponds  to  wave  length 
and  wave  propagation  in  the  optical  phenomena.  A 
medium  in  which  a  constant  magnetic  force  is  acting 
is  not,  in  consequence  of  that  force,  filled  with  waves 
travelling  in  one  direction,  as  when  light  is  propagated 
through  it.  The  only  resemblance  between  the  optical 
and  the  magnetic  phenomenon  is  that  at  each  point  of 
the  medium  something  exists  of  the  nature  of  an  an- 
gular velocity  about  an  axis  in  the  direction  of  the 
magnetic  force."  The  consideration  of  the  rotation  of 
the  plane  of  polarization  of  light  by  magnetic  force  led 
Maxwell  to  a  theory  of  magnetism  which  is  called  the 
hypothesis  of  molecular  vortices.  Since  there  is  good 
evidence  for  the  belief  that  there  is  some  kind  of  rota- 
tion going  on  in  the  magnetic  field,  Maxwell  investi- 
gated the  condition  of  motion  which  exists  when  a 
great  number  of  very  small  portions  of  matter  rotate 
on  their  own  axes,  these  axes  being  parallel  to  the  di- 
rection of  the  magnetic  force.  The  motion  of  these 
vortices  does  not  sensibly  affect  the  visible  motions  of 
large  bodies,  but  it  can  be  supposed  to  affect  the  peri- 
odic motion  of  the  medium  which  constitutes  the  phe- 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    273 

nomena  we  call  light.  According  to  this  theory,  the 
displacements  of  the  ether  will  produce  a  disturbance 
of  the  vortices,  and  this  disturbance  of  the  vortices  can 
be  supposed  to  react  on  the  ether,  and  in  this  way  can 
affect  the  propagation  of  light.  The  mathematical  dis- 
cussion of  this  theory  leads  to  the  following  conclusions : 

1.  Magnetic  force  is  the  effect  of  the  centrifugal 
force  of  the  vortices. 

2.  Electro-magnetic  induction   of   currents    is   the 
effect  of  the  forces  called  into  play  when  the  velocity 
of  the  vortices  is  changing. 

3.  Electro-motive  force  arises  from  the  stress  on  the 
connecting  mechanism. 

4.  Electric   displacement  arises    from    the    elastic 
yielding  of  the  connecting  mechanism.* 

While  thus  the  varied  manifestations  of  the  trans- 
formations of  energy  which  we  witness  in  the  subject 
of  electricity  apparently  compel  us  to  assume  the  exist- 
ence of  an  ether  pervading  all  space,  there  is  no  hy- 
pothesis of  physics  which  seems  more  arbitrary  than 
that  of  the  ether  to  the  cosmological  philosopher. 

The  Marquis  of  Salisbury,  hi  his  address  before  the 
British  Association,  Oxford,  1894,  says  of  the  ether : 

"  It  may  be  described  as  a  half -discovered  entity.  I 
dare  not  use  any  less  pedantic  word  than  entity  to  des- 
ignate it,  for  it  would  be  a  great  exaggeration  of  our 
knowledge  if  I  were  to  speak  of  it  as  a  body  or  even  a 
substance.  When,  nearly  a  century  ago,  Young  and 
Fresnel  discovered  that  the  motions  of  an  incandescent 
particle  were  conveyed  to  our  eyes  by  undulations,  it 
followed  that  between  our  eyes  and  the  particle  there 
must  be  something  to  undulate.  In  order  to  furnish 

*  Maxwell's  Electricity  and  Magnetism,  vol.  ii,  §  831. 


274  WHAT  IS  ELECTRICITY! 

that  something,  the  notion  of  the  ether  was  conceived, 
and  for  more  than  two  generations  the  main,  if  not  the 
only,  function  of  the  word  ether  has  been  to  furnish  a 
nominative  case  to  the  verb  '  to  undulate.'  Lately  our 
conception  of  this  entity  has  received  a  notable  exten- 
sion. One  of  the  most  brilliant  of  the  services  which 
Prof.  Maxwell  has  rendered  to  science  has  been  the 
discovery  that  the  figure  which  expressed  the  velocity 
of  light  also  expressed  the  multiplier  required  to  change 
the  measure  of  the  static  or  passive  electricity  into  that 
of  dynamic  or  active  electricity.  The  interpretation 
reasonably  affixed  to  this  discovery  is  that,  as  light  and 
the  electric  impulse  move  approximately  at  the  same 
rate  through  space,  it  is  probable  that  the  undulations 
which  convey  them  are  undulations  of  the  same  medium. 
And  as  induced  electricity  penetrates  through  every- 
tliing,  or  nearly  everything,  it  follows  that  the  ether 
through  which  its  undulations  are  propagated  must 
pervade  all  space,  whether  empty  or  full,  whether  oc- 
cupied by  opaque  matter  or  transparent  matter,  or  by 
no  matter  at  all.  The  attractive  experiments  by  which 
the  late  Prof.  Hertz  illustrated  the  electric  vibrations 
of  the  ether  will  only  be  alluded  to  by  me,  in  order  that 
I  may  express  the  regret  deeply  and  generally  felt  that 
death  should  have  terminated  prematurely  the  scientific 
career  which  had  begun  with  such  brilliant  promise  and 
such  fruitful  achievements.  But  the  mystery 'of  the 
ether,  though  it  has  been  made  more  fascinating  by 
these  discoveries,  remains  even  more  inscrutable  than 
before.  Of  this  all-pervading  entity  we  know  abso- 
lutely nothing  except  this  one  fact,  that  it  can  be  made 
to  undulate.  Whether  outside  the  influence  of  matter  on 
the  motion  of  its  waves  ether  has  any  effect  on  matter 
or  matter  upon  it,  is  absolutely  unknown.  And  even 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    275 

its  solitary  function  of  undulating  ether  performs  in  an 
abnormal  fashion  which  has  caused  infinite  perplexity. 
All  fluids  that  we  know  transmit  any  blow  they  have 
received  by  waves  which  undulate  backward  and  for- 
ward in  the  path  of  their  own  advance.  The  ether 
undulates  athwart  the  path  of  the  wave's  advance.  The 
genius  of  Lord  Kelvin  has  recently  discovered  what  he 
terms  a  labile  state  of  equilibrium,  in  which  a  fluid  that 
is  infinite  in  its  extent  may  exist,  and  may  undulate  in 
this  eccentric  fashion  without  outraging  the  laws  of 
mathematics.  ...  In  any  case  it  leaves  our  knowledge 
of  the  ether  in  a  very  rudimentary  condition.  It  has 
no  known  qualities  except  one,  and  that  quality  is  in 
the  highest  degree  anomalous  and  inscrutable.  The  ex- 
tended conception  which  enables  us  to  recognize  ethe- 
real waves  in  the  vibrations  of  electricity  has  added 
infinite  attraction  to  the  study  of  those  waves,  but  it 
carries  its  own  difficulties  with  it.  It  is  not  easy  to  fit 
in  the  theory  of  electrical  ether  waves  with  the  phe- 
nomena of  positive  and  negative  electricity ;  and  as  to 
the  true  significance  and  cause  of  those  counteracting 
and  complementary  forces  to  which  we  give  the  provi- 
sional names  of  negative  and  positive,  we  know  about 
as  much  as  Franklin  knew  a  century  and  a  half  ago." 

It  is  true  that  the  phenomena  presented  by  the  ac- 
tion of  two  electrified  pith  balls  are  still  full  of  mys- 
tery, but  not  more  so  than  many  phenomena  which  we 
perceive  daily  and  do  not  closely  examine.  One  will 
find  it  very  difficult,  for  instance,  to  explain  what  takes 
place  in  the  ether  when  we  light  a  candle.  We  can 
trace  the  waves  of  light  and  apply  our  mathematical 
processes  when  the  waves  are  investigated  at  some  dis- 
tance from  the  candle,  and  when  they  become  what  are 
called  plane  waves ;  but  when  we  endeavour  to  under- 
19 


276  WHAT  IS  ELECTRICITY  f 

stand  what  takes  place  at  a  point  of  light  when  the 
spherical  waves  receive,  so  to  speak,  their  primal  im- 
pulse, we  are  certainly  as  much  puzzled  as  we  are  to  ac- 
count for  the  action  of  two  electrified  pith  balls.  This 
certainly  can  be  maintained,  that  the  hypothesis  of  a 
medium  and  the  theory  of  action  from  point  to  point  in 
the  medium — the  theory  of  what  the  Germans  call 
Nahekrafte,  in  opposition  to  that  of  Fernkrafte,  or 
action  at  a  distance — has  enormously  increased  our  just 
conceptions  of  the  transformations  of  electrical  energy. 

The  hypothesis  of  action  at  a  distance  seems  at 
variance  with  the  modern  ideas  of  the  continuity  of 
matter.  It  was  naturally  suggested  by  the  attraction  be- 
tween the  moon  and  the  earth  and  the  attraction  of  the 
heavenly  bodies  in  general  with  regard  to  each  other. 
Perhaps  nothing  marks  so  strongly  the  modern  attitude 
toward  physical  manifestations  as  the  substitution  for 
action  at  a  distance  the  action  hi  matter  from  particle 
to  particle.  The  pole  of  a  magnet  does  not  attract  or 
repel  the  pole  of  another  magnet  by  a  direct  action 
through  space  which  is  not  influenced  by  the  matter  in 
this  space.  The  mutual  effect  of  the  poles  depends  on 
an  action  from  point  to  point  in  the  medium  between 
the  poles.  Every  magnetic  pole  sends  out  lines  of 
force  in  all  directions.  If  one  magnetic  pole  is  brought 
near  another  one,  the  force  of  attraction  or  repulsion 
between  them  immediately  becomes  manifest,  and  at 
the  same  time  the  disposition  of  the  lines  of  force  of  one 
magnetic  pole  is  altered  by  the  lines  of  force  of  the 
neighbouring  pole.  We  ordinarily  say  that  the  field  of 
force  of  one  pole  is  distorted  by  the  entrance  of  the 
other  pole. 

It  has  also  been  shown  that  there  is  a  rotary  action 
of  the  medium  near  the  poles  of  a  magnet.  This 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.     277 

rotary  action  is  from  point  to  point  in  the  medium  sur- 
rounding the  magnetic  poles,  and  it  undoubtedly  is  con- 
cerned in  the  attraction  phenomenon  which  two  pieces 
of  magnetized  steel  exhibit. 

Although  Faraday  and  Maxwell  advanced  the  study 
of  the  transf  ormations  of  energy  to  a  marked  degree 
by  the  conception  of  action  from  point  to  point  hi  place 
of  the  old  theory  of  action  at  a  distance,  it  seems  to  me 
that  even  between  the  ultimate  atoms  of  matter  we 
have  what  is  essentially  action  at  a  distance.  The  dis- 
tance may  be  infinitely  small,  still  it  is  only  small  rela- 
tively. 

The  theory  that  all  states  of  matter  fade  into  each 
other  by  insensible  degrees,  and  that  what  we  term 
atoms  are  merely  whirls  in  a  universal  medium,  enables 
us  by  a  forced  theory  to  escape  from  the  theory  of 
action  at  a  distance. 

Prof.  Emil  du  Bois-Reymond,  in  his  remarkable 
little  treatise  entitled  Die  Sieben  Weltrathsel  (The 
Seven  World  Mysteries  or  Puzzles),  says  :  "  A  physical 
atom — that  is,  a  body  which  is  disappearingly  small  in 
comparison  with  the  bodies  which  appeal  to  our  senses, 
yet,  in  spite  of  its  name,  a  still  divisible  mass  to  which 
physical  properties  and  conditions  of  movement  can  be 
ascribed  and  of  which  in  innumerable  numbers  larger 
masses  are  composed — is  an  extremely  useful  fiction,  es- 
pecially in  chemistry  and  in  the  mechanical  theory  of 
gases.  The  tendency,  however,  in  mathematical  physics 
is  to  shun  the  hypothesis  of  atoms  and  to  replace  the 
discrete  atom  by  the  volume  element  of  a  continuous 
medium." 

"  Newton  endeavoured  to  account  for  gravitation 
by  differences  of  pressure  in  an  ether,"  but  he  did  not 
publish  his  theory  "  because  he  was  not  able  from  ex- 


278  WHAT  IS  ELECTRICITY! 

periment  and  observation  to  give  a  satisfactory  account 
of  this  medium  and  the  manner  of  its  operation  in  pro- 
ducing the  chief  phenomena  of  nature."  The  phe- 
nomena of  light  also  demand  the  hypothesis  of  a 
medium.  ~No  one  has  better  summed  up  the  arguments 
for  the  existence  of  an  ether  than  Maxwell,  and  I  can 
not  do  better  than  give  his  principal  arguments  as  fol- 
lows :  * 

We  can  prove  by  means  of  the  phenomena  of  inter- 
ference that  light  is  not  a  substance.  If  light  from  a 
candle  is  divided  into  two  parts  which  are  made  to 
unite  after  traversing  two  different  paths  and  to  fall 
on  a  screen,  and  if  either  half  of  the  beam  is  shut  off 
by  a  screen,  the  other  half  will  still  illuminate  it.  If, 
however,  we  examine  the  light  on  the  screen  when  both 
portions  of  the  beam  are  allowed  to  fall  together  on 
it,  we  perceive  certain  dark  bands  crossing  the  screen. 
At  these  dark  points  the  waves  of  light  have  interfered 
with  each  other.  The  trough  of  one  wave  coincides 
with  the  crest  of  another  wave  and  darkness  results  ; 
or,  in  ordinary  language,  we  say  that  one  portion  of 
light  has  destroyed  another.  If  light  were  a  substance, 
one  portion  of  it  added  to  another  portion  could  not 
make  it  cease  to  be  evident  to  our  senses.  The  only 
explanation  of  the  interference  of  two  rays  of  light  that 
is  possible  is  this,  that  it  arises  from  a  periodic  move- 
ment in  a  medium. 

We  conclude  that  light  is  a  process,  and  not  a  sub- 
stance. The  medium  is  capable  of  transmitting  energy, 
as  is  seen  from  the  phenomena  of  electricity  and  of 
light.  This  energy  is  not  transmitted  instantly  from 
the  radiating  body  to  the  absorbing  body,  but  exists 

*  Encyclopaedia  Britannica,  Ether. 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    279 

for  a  time  in  the  medium.  Let  us  therefore  examine 
the  necessary  physical  properties  of  this  ether  :  The  co- 
efficient of  rigidity  of  ether  =  842  -S  ;  the  density  of  the 
ether  =  9'36  X  10~19 ;  the  coefficient  of  rigidity  of  steel 
is  about  8  X  1011 ;  and  that  of  glass,  2*4  X  1011.  It  has 
been  computed  that  if  the  temperature  of  the  atmos- 
phere were  everywhere  0°  C.,  and  if  it  were  in  equi- 
librium about  the  earth  supposed  to  be  at  rest,  its  den- 
sity at  an  infinite  distance  from  the  earth  would  be 
3  X  10-3*8,  which  is  about  3  X  10327  times  less  than  the 
estimated  density  of  the  ether.  In  the  regions  of  in- 
terplanetary space  the  density  of  the  ether  is  therefore 
very  great  compared  with  that  of  the  attenuated  at- 
mosphere of  interplanetary  space. 

Air  can  not  transmit  transverse  vibrations,  and  the 
normal  vibrations  which  the  air  transmits  in  the  case  of 
sound  travel  about  a  million  times  slower  than  light. 
"We  must  suppose  that  the  medium  (ether)  is  different 
from  the  transparent  media  known  to  us.  The  energy 
of  vibration  of  gross  particles  is  very  much  less  than  that 
of  the  ether ;  otherwise  a  much  greater  proportion  of 
incident  light  would  be  reflected  when  a  ray  passes 
from  vacuum  to  glass  or  from  glass  to  a  vacuum  than 
we  find. 

Faraday  says  :  "  For  my  own  part,  considering  the 
relation  of  a  vacuum  to  magnetic  force  and  the  general 
character  of  magnetic  phenomena  external  to  the  mag- 
net, I  am  much  more  inclined  to  the  notion  that  in  the 
transmission  of  the  force  there  is  such  an  action  exter- 
nal to  the  magnet,  than  that  the  effects  are  merely  at- 
traction and  repulsion  at  a  distance.  Such  an  action 
may  be  a  function  of  the  ether,  for  it  is  not  unlikely 
that,  if  there  be  an  ether,  it  should  have  other  uses  than 
simply  the  conveyance  of  radiation." 


280  WHAT  IS  ELECTRICITY? 

Tlie  following  are  the  objections  to  the  undulatory 
theory : 

1.  The  theory  indicates  the  possibility  of  undula- 
tions of  vibrations  normal  to  the  surface  of  the  waves. 
To  account  for  no  optical  effects  we  have  to  assume  the 
incompressibility  of  ether. 

2.  The  phenomena  of  reflection  are  best  explained 
on  the  hypothesis  that  the  vibrations  are  perpendicular 
to  the  plane  of  polarization ;  those  of  double  refraction 
require  us  to  assume  that  the  vibrations  are  in  that 
plane. 

3.  In  order  to  account  for  the  fact  that  in  a  doubly 
refracting  crystal  the  velocity  of  the  rays  in  any  principal 
plane  and  polarized  in  that  plane  is  the  same,  we  must 
assume  certain  highly  artificial  relations  among  the  co- 
efficients of  elasticity. 

Maxwell,  in  thus  stating  these  objections,  concludes 
with  the  following  defence  of  his  theory  :  "  The  electro- 
magnetic theory  satisfies  all  these  by  the  single  hypothe- 
sis that  the  electric  displacement  is  perpendicular  to 
the  plane  of  polarization.  No  normal  displacement  can 
exist;  and  in  doubly  refracting  crystals  the  specific 
dielectric  capacity  for  each  principal  axis  is  assumed  to 
be  equal  to  the  square  of  the  index  of  refraction  of  a 
ray  perpendicular  to  that  axis  and  polarized  in  a  plane 
perpendicular  to  that  axis.  Boltzman  has  found  that 
these  relations  are  approximately  true  in  the  case  of 
sulphur,  a  body  having  three  unequal  axes. 

"  Ether  transmits  transverse  vibrations  to  very  great 
distances  without  sensible  loss  of  energy  by  dissipation. 
A  molecular  medium,  moving  under  such  conditions 
that  a  group  of  molecules  once  near  each  other  remain 
near  each  other  during  the  whole  motion,  may  be  capa- 
ble of  transmitting  vibrations  without  much  dissipation 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    281 

of  energy ;  but  if  the  motion  is  such  that  the  groups  of 
molecules  are  not  merely  slightly  altered  in  configura- 
tion, but  entirely  broken  up  so  that  the  component 
molecules  pass  into  new  types  of  grouping,  then  in  the 
passage  from  one  type  to  another  the  energy  of  regular 
vibrations  will  be  frittered  away  into  heat.  ~We  can 
not  therefore  suppose  the  ether  like  a  gas. 

"  The  ether,  though  homogeneous  and  continuous, 
may  be,  as  regards  its  density,  rendered  heterogeneous  by 
motion  (Hypotheses  of  Vortex  Molecules,  Lord  Kelvin). 
Magnetic  influence  on  light  indicates  a  rotational  mo- 
tion of  the  media  when  magnetized.  This  motion  does 
not  imply  a  dissipation  of  energy. 

"  No  theory  of  the  ether  will  account  for  such  a  sys- 
tem of  molecular  vortices  being  maintained  for  an  in- 
definite time  without  their  energy  being  frittered  into 
heat." 

In  support  of  this  great  generalization  of  Maxwell, 
it  can  be  said  that  none  of  the  investigations  of  the 
great  army  of  physical  investigators  since  the  death  of 
Maxwell  tend  to  disprove,  but  rather  to  prove,  the  truth 
of  his  generalizations.  It  rests  upon  the  hypothesis  of 
the  ether ;  and,  as  we  have  seen,  the  various  transforma- 
tions of  energy  require  for  their  explanation  the  exist- 
ence of  a  medium.  Among  the  most  interesting  inves- 
tigations which  tend  to  prove  the  existence  of  the  ether 
are  those  in  Regard  to  the  cathode  rays.  The  word  ca- 
thode is  from  the  Greek  Kara,  down,  and  0809,  a  way. 
It  is  applied  to  the  negative  terminal  of  a  battery  or  to 
the  negative  terminal  of  a  Kuhmkorff  coil  or  trans- 
former. It  can  also  be  applied  to  the  outside  coating 
of  a  Leyden  jar,  the  ulterior  of  which  is  charged  posi- 
tively, the  outside  being  thus  charged  negatively. 
The  name  arose  from  the  old  and  earlier  supposition 


282  WHAT  IS  ELECTRICITY? 

that  electricity  flowed  from  tlie  positive  pole  down  to 
the  negative  pole. 

Now,  if  we  imbed  the  wires  connecting  the  cathode 
and  its  reverse,  the  anode,  of  a  Ruhmkorff  coil,  in  a  lit- 
tle glass  vessel  and  exhaust  the  vessel  of  air,  when  we 
excite  the  coil  and  when  we  reach  a  high  state  of  ex- 
haustion in  the  vessel  it  is  filled  with  luminosity,  and 
the  cathode  rays  can  be  recognised  as  streaming  out 
from  the  cathode. 

A  marked  peculiarity  of  the  cathode  rays  consists  in 
this,  that  they  are  independent  of  the  position  of  the 
anode,  and,  after  emerging  from  the  cathode,  they  con- 
tinue in  straight  lines  and  apparently  do  not  seek  the 
anode.  Thus,  if  both  cathode  and  anode  are  placed 
one  above  the  other  at  one  end  of  an  exhausted  tube, 
the  cathode  rays  continue  to  the  end  of  the  tube,  and 
do  not  bend  to  the  position  of  the  anode. 

Prof.  Crookes  believes  that  the  phosphorescent 
effects  produced  by  the  negative  rays  are  due  to  the 
impact  of  the  molecules  of  the  gas  on  the  phosphores- 
cent substances  which  emit  light.  It  is  still  undecided 
whether  the  luminous  appearance  is  due  to  such  im- 
pacts, or  whether  the  effect  is  entirely  electrical.  Prof. 
J.  J.  Thomson  points  out  that  the  phosphorescent  sub- 
stance on  which  these  cathode  rays  fall  is  subjected  to 
very  rapid  periodic  change  of  polarization,  which,  ac- 
cording to  the  electro-magnetic  theory  of  light,  would 
produce  the  same  effect  as  if  light  fell  on  the  phospho- 
rescent substance,  in  which  case  we  know  that  it  would 
phosphoresce. 

In  considering  the  remarkable  light  effects  produced 
by  currents  of  high  frequency,  we  perceive  that  we 
have  to  do  with  an  increased  activity  of  the  molecules 
of  a  rarefied  gas  which  is  produced  by  the  electrical  en- 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    283 

ergy  stored  up  in  the  medium  near  the  electrodes  of  our 
little  lamps.  Thus  we  have  a  bombardment  of  these 
electrodes  or  of  the  walls  of  the  inclosing  vessel ;  and 
these  walls  can  suitably  reflect  or  direct  these  molecules, 
as  we  have  seen  in  Prof.  Crookes's  experiment.  The 
cathode  rays  may  be  considered  as  a  radiation  of  elec- 
tric energy,  which  is  made  visible  in  rarefied  media  and 
which  can  also  be  detected  outside  such  media.  The 
visible  transformation  of  electric  energy  from  the  elec- 
trode, termed  the  cathode,  can  be  made  to  pass  through 
metal  walls,  and  can  be  seen  outside  these  metal  walls. 
This  phenomenon  is  one  which  at  present  is  attracting 
perhaps  more  attention  from  scientific  men  than  any 
other  in  electricity,  for  in  this  phenomenon  we  see  a 
manifestation  in  the  medium  which  may  lead  to  a 
better  understanding  of  the  polarization  of  the  ether. 

Before  describing  more  minutely  the  interesting 
apparatus  by  which  this  phenomenon  is  studied,  let  us 
grasp  the  salient  facts.  The  cathode  rays  can  be  made 
to  pass  through  substances  which  are  entirely  opaque  to 
ordinary  light.  We  can,  so  to  speak,  see  what  may  be 
termed  an  electrical  candle  through  a  wall,  for  these 
cathode  rays  can  be  made  to  pass  through  sheets  of 
aluminium,  and  gold  and  silver,  and  many  other  opaque 
substances  which  cut  off  ordinary  light  entirely. 

The  sheets  of  aluminium  which  allow  these  electric- 
light  rays  to  pass  and  cut  off  ordinary  rays  is  0*00265 
millimetre  (about  -^^  of  an  inch)  thick.  The  rays 
are  perfectly  visible  in  the  open  air  of  a  dark  room 
after  they  have  passed  into  it  from  a  rarefied  tube 
through  such  thin  opaque  metallic  sheets.  The  rays 
spread  out  in  all  directions.  They  excite  phosphores- 
cent bodies,  such  as  uranium  glass,  to  a  brilliant  glow, 
and  blacken  photographic  plates. 


284:  WHAT  IS  ELECTRICITY? 

Lenard  has  described  the  phenomenon  of  cathode 
rays  very  fully,  and  has  investigated  it  with  an  appa- 
ratus which,  in  its  essentials,  consisted  of  a  Oookes 
tube  (Fig.  28,  page  193)  provided  with  a  window. 

The  window  is  hermetically  closed  by  a  thin  alu- 
minium sheet  through  which  the  rays  pass  into  the 
outer  room,  and  the  whole  apparatus  is  inclosed  by  a 
metallic  vessel,  which  is  connected  to  the  ground  to 
lead  off  disturbing  electric  charges.  A  quartz  plate  one 
half  millimetre  thick,  placed  between  the  window  and 
a  phosphorescing  body,  extinguishes  the  light.  Thin 
plates  of  aluminium,  gold,  and  silver  allow  the  light 
to  pass  through.  The  light  appears  between  such 
plates  and  the  window,  and  also  beyond  the  plates, 
while  the  plates  themselves  remain  dark.  The  opacity 
of  the  quartz  plates  and  the  transparency  of  the  metal 
plate  to  these  rays  form  the  marked  difference  between 
the  cathode  rays  and  the  light  rays.  It  is  probable, 
however,  that  all  substances  in  sufficiently  thin  sheets 
are  transparent  to  the  cathode  rays.  Soap  films 
stretched  on  wire  supports  cut  off  the  rays  when  they 
are  thicker  than  0'0012  millimetre.  Aluminium  plates, 
however,  0-027  millimetre  thick  allow  the  rays  to  pass. 

-In  the  case  of  light,  one  body  in  a  layer  one  hun- 
dred millionth  of  a  metre  thick  can  be  more  opaque 
than  another  body  a  metre  thick.  Such  enormous  dif- 
ferences, however,  do  not  exist  in  the  case  of  the  cathode 
rays.  The  ordinary  atmosphere  is  an  unfit  medium  for 
the  passage  of  the  cathode  rays,  for  they  speedily  lose 
in  it  their  movement  in  a  straight  line  and  become 
greatly  diffused.  The  rays  from  the  •  cathode  window 
are  diffused  very  much  as  the  rays  of  sunlight  are  diffused 
in  passing  from  a  slit  or  opening  into  milk  and  water. 

No  heating  effect  of  the  cathode  rays  has  been 


THE  ELECTRO-MAGNETIC  THEORY  OF  LIGHT.    285 

detected.  Lenard  placed  a  delicate  thermal  junction  in 
the  rays  which  showed  no  heat  effects,  while  a  candle  at 
the  distance  of  fifty  centimetres  gave  with  the  same 
thermal  junction  a  marked  effect. 

It  was  noticed  that  electrified  bodies  lose  their  charges 
when  the  cathode  rays  fall  upon  them,  or  when  they  are 
placed  in  the  neighbourhood  of  the  window  through 
which  the  rays  pass.  It  is  well  known  that  when  the 
exhaustion  in  the  tubes  in  which  the  cathode  rays  are 
produced  is  pushed  to  an  extreme,  so  that  the  vacuum 
is  well-nigh  perfect,  the  rays  can  no  longer  be  produced, 
and,  in  fact,  all  electrical  manifestations  visible  as  light 
phenomena  disappear.  The  vacuum  seems  to  afford  an 
infinite  resistance  to  electricity.  To  test  the  question 
whether  the  cathode  rays  could  travel  in  a  vacuum,  al- 
though they  could  not  be  excited  in  it,  Lenard  arranged 
a  tube  1*50  metre  long  (4r'92  feet),  into  which  the  cath- 
ode rays  could  pass  from  the  rarefied  tube  in  which  they 
were  generated.  This  long  tube  was  then  made  as  per- 
fect a  vacuum  as  modern  methods  permit.  The  pres- 
sure of  the  air  remaining  in  the  tube  was  only  0*000009 
millimetre  mercury,  or  O'Ol  X 10''  of  an  atmosphere. 

The  cathode  rays  streamed  from  the  cathode  through 
the  window  into  the  long  cylinder  which  inclosed  the 
vacuum.  They  were  not  visible  until  they  struck  a 
little  phosphorescent  screen,  which  could  be  moved  along 
the  tube  to  different  distances  by  means  of  a  magnet, 
there  being  a  bit  of  iron  on  the  screen.  The  rays  trav- 
elled in  straight  lines  in  the  vacuum,  and  could  be  de- 
tected at  the  end  of  the  tube,  1*50  metre  from  the 
window.  Lenard  remarks  that  the  ether  is  therefore 
the  medium  by  which  the  rays  travel  and  in  which  they 
manifest  their  peculiar  phenomena.  The  motions  in  the 
ether  must  be  of  such  extremely  minute  order  that  the 


286  WHAT  IS  ELECTRICITY! 

size  of  the  molecules  is  of  relative  importance.  The 
molecules  of  gas  muddle,  so  to  speak,  the  ether,  but  it  is 
noticeable  that  it  is  only  the  mass  of  the  molecule  which 
influence  the  phenomena. 

If  one  could  measure  the  velocity  of  the  cathode  rays 
in  the  ether,  and  observe  their  refraction  by  different 
media,  one  could  connect  the  phenomena  still  more 
closely  with  ordinary  light  waves.  It  seems  as  if  the 
study  of  such  phenomena  in  the  ether  is  destined  to 
greatly  increase  our  knowledge  of  the  relations  between 
the  various  forms  of  energy.  Eays  similar  to  the  cath- 
ode rays  could  pass  from  the  sun  to  the  earth  through 
the  ether  of  space  and  exercise  an  effect  in  our  atmos- 
phere, although  they  would  be  invisible  in  the  vacuum 
which  exists  in  space  between  the  sun  and  the  earth.* 

The  electro-magnetic  theory  of  light  demands  that  a 
vacuum  should  be  a  nonconductor  of  electricity  ;  for  if 
it  were  a  good  conductor,  it  would  be  opaque  to  the  elec- 
tric waves,  and,  according  to  the  electro-magnetic  theory, 
no  light  would  come  to  us  from  the  sun.  In  experi- 
ments with  the  cathode  rays  we  find  that  they  can  pass 
through  certain  conductors  in  thin  layers.  They  are 
cut  off,  however,  by  layers  of  appreciable  thickness. 

*  Ann.  der  Physik  und  Chemie,  51,  1894,  p.  225. 


CHAPTER  XX. 

THE   X   KAYS. 

SINCE  the  writing  of  the  previous  chapter  interest 
in  the  remarkable  phenomena  of  the  cathode  rays  has 
been  reawakened  to  a  marked  degree  by  the  discovery 
of  Prof.  Rontgen,  who,  by  the  use  of  ordinary  dry 
plates  and  without  the  use  of  an  aluminium  window, 
has  taken  photographs  through  wood  and  through  the 
human  hand  by  means  of  what  he  terms  the  X  rays, 
which  he  supposes  are  excited  either  in  the  glass  walls 
of  the  Crookes  tube  or  in  the  media  outside  the  tube 
by  means  of  the  cathode  rays. 

We  see,  therefore,  that  the  literature  of  the  subject 
must  be  sought  in  the  papers  of  Hittorf ,  Crookes,  Hertz, 
Lenard,  and  Rontgen ;  and  the  interest  in  the  mysteri- 
ous manifestations  of  these  invisible  rays  is  twofold: 
first,  in  regard  to  the  possible  application  of  the  phe- 
nomena to  surgery,  since  the  rays  show  a  specific  ab- 
sorption, passing  more  easily  through  the  flesh  than 
through  bones  or  glass  or  metallic  particles ;  and,  sec- 
ondly, in  relation  to  the  questions  whether  we  are  deal- 
ing here  with  radiant  matter  shot  forth  from  the  nega- 
tive pole  or  cathode  or  with  longitudinal  waves  of 
electricity. 

The  term  cathode,  we  have  seen,  is  applied  to  the 
zinc  pole  or  negative  pole  of  an  ordinary  battery.  It 


288  WHAT  IS  ELECTRICITY? 

is  that  terminal  of  an  electrical  machine  which  glows 
least  in  the  dark  when  the  machine  is  excited.  It  is 
the  shortest  carbon  in  the  ordinary  street  electric  lamp. 
The  positive  carbon  or  anode  burns  away  twice  as  fast 
as  the  negative  carbon  or  cathode.  If  the  electric  light 
is  formed  in  a  high  vacuum  by  means  of  a  great  elec- 
tro-motive force,  we  no  longer  have  a  voltaic  arc  or  a 
spark ;  instead  of  this  the  exhausted  vessel  is  filled  with 
a  feeble  luminosity,  and  a  beam  of  bluish  rays  is  seen 
to  stream  from  the  negative  terminal  or  cathode.  When 
these  rays  strike  the  glass  walls  of  the  vessel  they  excite 
a  strong  fluorescence.  If  the  glass  contains  an  oxide 
of  uranium  this  fluorescence  is  yellow ;  if  it  contains  an 
oxide  of  copper  it  is  green.  Rontgen  supposes  that 
this  fluorescence  excited  by  the  cathode  rays  is  con- 
nected in  some  way  with  the  formation  of  what  he 
terms  the  X  rays.  Now,  a  photograph  of  the  bones  in 
the  hand,  for  instance,  can  be  obtained  by  placing  a 
sensitive  plate  in  an  ordinary  photographic  plate-holder, 
and  by  resting  the  hand  on  the  undrawn  slide  in  the  day- 
light, with  the  palm  of  the  hand  outward  and  toward  the 
cathode,  and  about  six  inches  away  from  it ;  the  bones 
of  the  hand  are  thus  brought  in  the  nearest  possible 
position  to  the  sensitive  plate.  At  the  time  of  the 
present  writing,  the  breast  and  the  abdomen  of  the  hu- 
man body  present  too  great  thickness  for  successful 
photographs,  and  the  attempts  to  obtain  representations 
of  the  cavity  in  which  the  brain  is  situated  have  been 
failures,  since  the  rays  do  not  show  any  marked  differ- 
ence in  fleshy  tissues.  Nothing  can  be  obtained  in 
these  attempts  to  photograph  the  brain  but  a  contour 
of  the  cavity  in  which  it  is  situated,  and  possibly  a 
shadowy  representation  of  a  bullet  which  might  be  im- 
bedded in  the  head.  The  method  of  obtaining  a  sue- 


THE  X  RAYS.  289 

cessful  photograph  of  the  hand  shows  the  present  lim- 
itations of  the  method.  In  order  to  obtain  a  fairly 
sharp  shadow  of  a  bone  or  of  a  shot,  it  should  not  be 
more  than  an  inch  away  from  the  sensitive  plate.  The 
term  shadow,  however,  is  somewhat  misleading.  The 
photograph  of  the  hand  by  the  X  rays  is  entirely 
different  from  one  produced  by  resting  the  hand  in  a 
similar  position  to  that  above  described  against  an  un- 
covered sensitive  plate  in  a  dark  room  and  then  lighting 
a  match.  By  the  last  method  we  should  obtain  a  true 
shadow  of  the  hand,  the  flesh  would  throw  as  dense  a 
shadow  as  the  bones,  and  the  latter  could  not  be  de- 
tected in  the  general  blackness.  In  the  cathode  photo- 
graph, on  the  other  hand,  a  difference  in  absorptive 
power  is  shown :  the  flesh  looks  like  a  hazy  film  around 
the  skeleton,  and  even  the  medulla  cavities  can  be  made 
out,  and  the  varying  thickness  of  the  bones  is  more  or 
less  shown.  This  specific  absorption  is  of  great  scien- 
tific interest  as  well  as  of  practical  importance. 

Now,  these  X  rays  will  penetrate  several  inches  of 
wood,  with  varying  amount  of  absorption,  but  they  are 
almost  entirely  cut  off  by  glass  as  thick  as  a  window 
pane.  They  pass  through  thin  layers  of  aluminium, 
even  layers  as  thick  as  a  silver  ten-cent  piece,  while  the 
silver  coin  almost  entirely  intercepts  them. 

It  therefore  immediately  occurs  to  one,  Why  not 
return  to  Lenard's  tube,  provide  a  Crookes  tube  with 
an  aluminium  window,  and  thus  save  the  great  absorp- 
tion of  the  glass  walls  of  the  tube  ?  There  are  certain 
practical  difficulties  in  the  way.  The  aluminium  must 
be  very  thin.  Lenard  used  a  window  which  we  have 
seen  was  about  one  nine  thousandth  of  an  inch  thick, 
and  it  was  necessarily  very  small,  in  order  to  stand  the 
atmospheric  pressure.  An  aluminium  window  one  eighth 


290  WHAT  IS  ELECTRICITY? 

of  an  inch  thick,  or  as  thick  as  a  ten-cent  piece,  would 
absorb  nearly  as  much  as  the  glass  walls  of  the  present 
forms  of  Crookes  tubes,  which  are  not  more  than  one 
sixtieth  of  an  inch  thick.  Glass  vessels  seem  at  pres- 
ent to  be  more  practical  than  any  composite  form,  in 
which  aluminium  is  glued  to  a  glass-supporting  vessel : 
first,  because  they  can  be  blown  very  thin,  and  in  a  shape 
strong  enough  to  withstand  the  atmospheric  pressure ; 
secondly,  because  the  occluded  air  can  be  more  effect- 
ively driven  off  the  inner  walls  of  the  vessels  by  heat- 
ing it  while  it  is  being  exhausted  than  it  can  be  expelled 
from  a  vessel  of  any  other  material. 

To  obtain  successful  photographs,  the  exhaustion  of 
the  air  must  be  pushed  to  a  high  degree.  Moreover, 
a  high  electro-motive  force  is  necessary.  Pictures 
can  be  taken  in  one  second  of  the  skeleton  of  the  hu- 
man hand  by  means  of  high  vacua  tubes  excited  by 
high  electro-motive  force.  Even  in  this  bare  recital  of 
the  present  limits  of  the  application  of  the  X  rays  to 
photography  we  perceive  great  possibilities  in  the  ap- 
plication of  the  method  to  the  surgery  of  the  human 
extremities.  There  is  no  doubt  that  small  foreign 
bodies,  like  shot  and  pieces  of  glass,  can  be  detected 
in  the  fleshy  tissues  of  the  hand.  Certain  accessible 
regions  of  the  body,  like  the  mouth,  can  possibly  be 
examined  by  placing  a  sensitive  film  inside  the  mouth 
and  the  cathode  outside  of  the  cheek ;  and  it  does  not 
seem  improbable  that  a  suitable  cathode  vessel  can  be 
inserted  into  certain  abdominal  regions  and  a  photo- 
graph be  obtained  by  placing  a  sensitive  plate  on  the 
outside  of  the  body.  I  have  shown  that,  by  employing 
two  cathodes  at  the  proper  distance  apart,*  stereoscopic 

*  American  Journal  of  Science.  March,  1896. 


THE  X  RAYS. 


291 


representations  of  the  bones  can  be  obtained,  and  an 
estimate  formed  of  the  position  of  foreign  bodies. 

It  seems  to  be  now  well  established  that  the  radia- 
tions characteristic  of  the  X  rays  proceed  from  the  solid 
body  upon  which  the  cathode  rays  impinge.  One  of 
the  most  successful  forms  of  Crookes  tubes  for  pro- 
ducing, the  Kontgen  photographs  is  called  a  focus  tube 
(devised  in  the  chemical  laboratory  of  King's  College, 
London).  It  consists  of  a  tube  similar  to  that  repre- 
sented in  Fig.  28.  The  cathode  rays  impinge  on  a 
thin  plate  of  platinum  constituting  the  anode,  which  is 
inclined  forty-five  degrees  to  the  axis  of  the  concave 
mirror.  Prof.  Elihu  Thomson  has  devised  a  double- 


FIG.  53. 


focus  tube  which  consists  of  two  concave  mirrors,  A 
and  B ;  a  Y-shaped  reflector,  C,  is  placed  at  their  com- 
mon focus  (Fig.  53).  This  tube  is  adapted  for  the  use 
of  alternating  currents.  With  a  Thomson  and  Tesla 
coil,  practically  instantaneous  photographs  can  be  taken 
of  objects  which  are  placed  close  to  the  photographic 
plate.  The  method  of  studying  the  effects  of  the 
X  rays  by  means  of  fluorescent  screens  is  more  expedi- 
tious than  that  of  photography.  A  fluorescent  screen 
is  simply  a  sheet  of  pasteboard  covered  with  a  fluorescent 
substance.  Edison  has  discovered  that  crystallized  tung- 
20 


292  WHAT  IS  ELECTRICITY? 

state  of  calcium  is  highly  fluorescent.  A  pasteboard 
covered  with  this  substance  forms  the  closed  end  of 
a  box  into  which  one  looks,  the  hand  or  limb  being 
pressed  against  the  outside  of  the  pasteboard  screen, 
the  fluorescent  substance  being  on  the  side  at  which 
one  looks — that  is,  inside  the  box.  By  means  of  such 
a  fluoroscope  one  can  see  the  shadow  of  one's  hand 
after  the  X  rays  have  passed  through  several  doors, 
and  at  a  distance  of  at  least  fifteen  feet  from  the 
Crookes  tube.  Sensitive  photographic  plates  are 
fogged  through  brick  walls  a  foot  thick. 

Let  us  now  return  to  some  of  the  interesting  scien- 
tific questions  which  have  arisen  in  regard  to  this  ap- 
parently new  manifestation  of  the  cathode  rays.  In  the 
first  place,  they  are  apparently  not  refracted  by  paraf- 
fin, vulcanite,  or  wood,  or  by  any  substance  which  is 
penetrated  by  them.  To  test  this,  I  employed  a  double- 
convex  lens  of  wood,  and  also  a  double-concave  lens  of 
the  same  material.  I  placed  two  copper  rings  in  the 
concavity  of  the  double-concave  lens  of  wood,  and  also 
a  similar  copper  ring  outside  the  lens  at  the  same  height 
from  the  sensitive  plate  as  one  of  the  rings  which  rested 
on  the  wood  of  the  lens.  I  also  placed  a  ring  on  the 
double-convex  lens,  and  employed  two  cathodes  to  ob- 
tain two  shadows  from  different  positions.  The  thick- 
ness of  the  wooden  lenses  varied  from  half  an  inch  to 
three  quarters  of  an  inch.  The  images  obtained  through 
the  wood  of  the  lenses  were  not  distorted  or  changed 
in  figure  in  any  way  by  the  wood,  and  therefore  no  re- 
fraction could  be  observed  by  this  method.  On  account 
of  the  quick  diffusibility  of  the  rays,  no  accurate  meth- 
od of  determining  a  possible  index  of  refraction  seems 
possible.  If  the  photographic  effect  is  due  to  longitu- 
dinal waves  in  the  ether,  and  if  these  waves  travel  with 


THE  X  RAYS.  293 

great  velocity,  no  refraction  would  probably  be  ob- 
served. Maxwell's  electro-magnetic  theory  of  light  sup- 
poses that  only  transverse  waves  are  set  up  in  the  ether, 
and  no  longitudinal  waves  exist.  On  the  other  hand, 
Helmholtz's  electro-magnetic  theory  of  light  postulates 
longitudinal  waves  as  well  as  transverse  waves.  The 
longitudinal  waves  travel  with  an  infinite  velocity.  Is 
it  therefore  possible  that  the  X  waves  are  the  longitu- 
dinal waves  of  Helmholtz's  theory  ?  Our  apparent  in- 
ability to  refract  the  rays  lends  colour  to  this  hypothesis. 
Kontgen,  in  the  preliminary  account  of  his  experiments, 
intimates  that  the  phenomena  may  be  due  to  longitu- 
dinal waves  ;  and  in  a  late  article  by  Jaumann,  entitled 
Longitudinal  Light,*  Maxwell's  electro-magnetic  equa- 
tions are  modified  so  as  to  embrace  the  phenomenon  of 
cathode  rays;  and  the  author  shows  that  even  Max- 
well's theory  can,  under  certain  conditions,  give  a  longi- 
tudinal wave. 

The  cathode  rays  can  be  deflected  by  a  magnet,  and 
it  is  said  that  the  X  rays  can  not.  It  must  be  borne  in 
mind,  however,  that  when  the  cathode  rays  are  widely 
divergent  it  is  difficult  to  deflect  them  by  a  magnet ; 
the  stream  density,  so  to  speak,  is  too  feeble.  The  X 
rays,  therefore,  may  be  only  cathode  rays  modified  by 
passing  through  the  glass  vessel ;  and  the  stream  of 
rays  may  be  of  too  feeble  a  character  to  be  influenced  by 
a  magnet — that  is,  they  may  be  still  cathode  rays.  The 
want  of  refractive  power  and  the  want  of  magnetic  ac- 
tion have  not  been  fully  established.  The  electrostatic 
lines  of  force  go  out  from  a  charged  conductor  at  right 
angles  to  the  surface  of  the  conductor.  I  have  had 
constructed  a  Crookes  tube  with  two  parallel  terminals 

*  Annalen  der  Physik  und  Chemie,  No.  1,  1896. 


294  WHAT  IS  ELECTRICITY! 

of  aluminium.  The  fluorescence  in  the  walls  of  the 
vessel,  when  it  was  exhausted,  showed  that  the  cathode 
rays  went  out  from  every  element  of  the  cathode  at  right 
angles  to  it.  By  bending  the  cathode  into  an  arc  of  a 
circle  the  cathode  beams  travelled  over  the  surface  of  the 
vessel,  forming  zones  of  light  the  centres  of  which  were 
in  the  bent  wire.  Is  it  not  possible  that  by  the  electro- 
static action  the  few  molecules  of  air  left  in  the  high 
vacua  are  shot  off  with  great  velocity  and  bombard 
the  walls  of  the  vessel,  and  thus  give  rise  to  the  fluo- 
rescent light,  and  also  to  an  agitation  of  the  molecules 
of  matter  outside  the  vessel  ?  This  may  be  called  the 
molecular  view  of  the  phenomenon.  I  confess  it  is 
difficult  to  see  why  the  molecular  agitation  is  stopped 
by  a  thin  sheet  of  glass  and  not  by  an  inch  of  wood. 
It  is  certain  that  a  few  molecules  must  be  left  in  the 
high  vacua,  for  the  cathode  rays  can  not  be  formed  in 
a  perfect  vacuum. 

It  is  also  true  that  it  is  useless  to  attempt  to  obtain 
photographs  in  any  reasonable  time  from  tubes  which 
do  not  show  a  strongly  marked  cathode  beam,  or  from 
tubes  which  on  reversing  the  electric  current  through 
them  do  not  show  a  marked  difference  between  the 
light  at  the  cathode  and  that  at  the  anode.  In  poorly 
exhausted  tubes  one  can  perceive  a  faint  appearance  of 
a  cathode  beam,  which  is  lost  at  a  short  distance  from 
the  cathode,  as  if  the  molecules  which  are  shot  off  meet 
with  such  a  crowd  of  more  slowly  moving  ones  that 
their  energy  is  soon  lost,  and  the  cathode  beam  is 
quickly  diffused  like  a  beam  of  sunlight  passing  into 
milk  and  water.  Thus  the  beam  of  cathode  or  X  rays 
emerging  from  the  glass  vessel  into  the  air  is  soon  no 
longer  conical  in  form.  The  sides  of  the  cone  of  rays 
are  no  longer  straight;  they  are  curved,  as  if  the  gen- 


THE  X  RAYS.  295 

eratrix  of  the  cone  were  a  curved  line  instead  of  a 
straight  line,  and  the  beam  is  soon  lost  in  a  turbid 
medium.  One  can  imagine  a  stream  of  projectiles  be- 
ing similarly  dispersed  in  striving  to  pass  into  a  region 
of  sluggishly  moving  shot.  This  molecular  view  of  the 
phenomenon  seems  at  first  sight  to  be  a  more  tangible 
one  than  the  longitudinal  wave  theory.  Yet  the  amount 
of  energy  required  by  any  corpuscular  theory  would 
seem  to  be  enormous.  It  is  possible,  too,  that  the  im- 
pact of  the  molecules  on  the  aluminium  window  of 
Lenard,  or  on  the  glass  sides  of  the  vessel,  may  serve 
to  start  ripples,  so  to  speak,  in  the  ether,  which  are 
propagated  with  the  velocity  of  light. 

The  Eontgen  phenomenon  seems  to  be  a  manifesta- 
tion of  cathode  rays  brought  to  light  and  endowed 
with  great  practical  interest  by  its  application  to  dry- 
plate  photography.  When  we  return  to  the  classical 
investigation  of  Lenard  mentioned  in  the  last  chapter, 
we  are  impressed  by  his  apparently  crucial  experiment 
which  he  describes  in  regard  to  the  existence  of  an 
ether  or  medium.  Energy  passed  into  the  vacuum  he 
formed,  and  could  be  detected  from  point  to  point. 
We  can  conceive  of  its  passing  through  the  ether  in 
the  tube  by  a  wave  motion,  but  not  by  a  molecular 
movement,  for  there  were  no  molecules  to  move.  The 
molecular  bombardment  must  have  stopped  at  the 
aluminium  window,  and  the  resulting  energy  may  have 
been  propagated  by  ripples  in  the  ether.  This  experi- 
ment of  Lenard  seems  to  me  the  most  interesting  one 
in  the  subject  of  cathode  rays.  The  greatest  mystery, 
however,  which  envelops  the  subject  is  the  action  of  the 
X  rays  on  bodies  charged  with  electricity.  When  the 
rays  fall  on,  for  instance,  a  charged  pith  ball,  the 
charge  disappears.  Prof.  J.  J.  Thomson  and  Prof. 


296  WHAT   IS  ELECTRICITY? 

Rhigi  have  found  that  a  positive  as  well  as  a  negative 
charge  is  dispelled  by  the  X  rays.  The  energy  of  the 
medium  about  the  pith  ball  is  changed  to  a  marked  de- 
gree, and  in  this  phenomenon  we  seem  to  be  brought 
closer  to  a  wave  theory  in  a  medium  than  to  a  molecu- 
lar theory  of  movement  of  matter. 

The  tendency  at  present  is  to  believe  that  the  X 
rays  are  waves  of  ultra-violet  light  of  much  smaller  di- 
mensions than  any  that  have  been  hitherto  detected. 
D.  A.  Goldhammer*  strongly  advocates  this  view. 
Prof.  Rontgen's  reasons  for  believing  that  the  new 
radiations  discovered  by  him  were  not  those  of  ultra- 
violet light  were  as  follows : 

a.  The  X  rays  suffer  no  refraction  in  passing  from 
air  to  water,  bisulphide  of  carbon,  aluminium,  rock  salt, 
glass,  zinc,  etc. 

b.  They    are    not    regularly    reflected    by   known 
bodies. 

c.  They  can  not  be  polarized  by  known  means. 

d.  The  density  of  a  body  apparently  influences  their 
absorption  more  than  that  of  any  other  factor. 

If  the  X  rays  are  very  short  transverse  waves  of 
light  which  are  too  small  in  comparison  with  uneven - 
ness  of  highly  polished  substances  to  be  regularly  re- 
flected or  polarized,  b  and  c  can  be  explained. 

When  we  consider  also  the  phenomenon  of  anoma- 
lous refraction  and  dispersion  the  behaviour  of  the  so- 
called  X  rays  is  not  so  remarkable.  Certain  substances, 
like  fuchsin  and  aniline,  exhibit  anomalous  refraction — 
that  is,  a  ray  of  blue  light  may  be  more  refracted  in 
passing  through  a  solution  of  these  substances  than  a 
ray  of  violet  light;  while  with  substances  like  glass, 

*  Annalen  der  Physik  und  Chemie,  No.  4,  1896. 


THE  X  RAYS.  297 

which  exhibit  normal  refraction,  the  violet  rays  are 
more  refracted  than  the  blue  rajs. 

In  certain  cases  of  anomalous  refraction  and  disper- 
sion the  amount  of  refraction  (index  of  refraction)  di- 
minishes as  the  length  of  wave  grows  shorter.  Gold- 
hammer  therefore  concludes  that  a  and  c  can  be  thus 
explained  by  anomalous  refraction  and  dispersion,  to- 
gether with  the  hypothesis  that  the  X  rays  are  ordi- 
nary transverse  vibrations  of  the  ether,  such  as  consti- 
tute ordinary  ultra-violet  light.  The  wave  lengths  of 
the  X  rays  is,  however,  much  smaller  than  those  of  any 
hitherto  observed  ultra-violet  rays. 

In  1867  M.  Boussinesq  presented  a  paper  to  the 
French  Academy  on  the  Theorie  nouvelle  des  ondes 
lumineuses,*  in  which  the  effects  of  the  momentum  com- 
municated to  the  molecules  of  matter  by  the  ether  are 
considered  to  be  the  cause  of  reflection,  refraction,  po- 
larization, dispersion,  etc.  The  ether  is  supposed  to  be 
homogeneous  and  of  the  same  density  and  rigidity  in  all 
bodies,  and  that  when  light  enters  a  transparent  medium 
the  molecules  of  that  medium  may  be  set  in  motion  isoch- 
ronously  with  the  motion  of  the  ether.  Sellmeyer  also,  in 
1872,  adopted  the  hypothesis  that  the  ponderable  atoms 
vibrate,  but  with  much  smaller  amplitudes  than  the 
ether  particles.  The  theories  of  Boussinesq  and  Sell- 
meyer lead  to  expressions  for  indices  of  refraction  in 
cases  of  anomalous  refraction  which  bear  upon  the  X- 
ray  phenomena.  The  electrical  stress  acting  on  the 
ether  may  probably  serve  to  set  the  molecules  of  the 
fluorescent  substances  into  their  peculiar  rates  of  vibra- 
tion. 

*  Glazebrook,  Optical  Theories,  British  Association  Report,  1885. 


CHAPTER  XXI. 

THE   SUN. 

IN  asking  ourselves  What  is  electricity  ?  we  are  natu- 
rally led  to  inquire  into  the  constitution  of  the  sun,  to 
which  we  owe  our  electrical  energy.  What,  therefore, 
is  the  constitution  of  the  sun  ?  It  seems  strangely  anal- 
ogous to  an  enormous  electrical  furnace.  If  one  gazes 
into  an  electrical  furnace  in  which  there  is  a  mass  of 
molten  metal — silver,  for  instance — one  perceives  va- 
pours shifting  over  the  glistening  mass.  In  this  f  urnace 
carbon  becomes  freed  from  its  impurities ;  the  iron  and 
sodium,  for  instance,  are  driven  off  in  vapour,  and  the 
pure  carbon  lies  hi  the  heart  of  the  furnace,  surrounded 
by  clouds  of  what  once  existed  throughout  its  mass. 

When  one  gazes  at  the  spectrum  of  the  sun  one 
marvels  at  the  mysterious  arrangement  of  the  dark  lines 
which  indicate  the  absorption  of  the  vapour  of  some 
metal.  According  to  the  electro-magnetic  theory  of 
light,  these  dark  lines  represent  an  absorption  of  electric 
energy  also.  To  what  is  due  the  electric  energy  which 
reaches  us  in  electro-magnetic  waves  propagated 
through  the  infinite  space  between  us  and  the  farthest 
star? 

The  spectrum  of  the  sun  is  like  some  ancient  pa- 
limpsest, with  inscription  upon  inscription  laid  upon 
each  other.  A  photograph  of  it  is  a  composite  photo- 

203 


THE  SUN.  299 

graph  made  up  of  the  effects  of  the  vapour  of  iron  and 
calcium,  of  cobalt  and  nickel,  of  sodium,  and  many  other 
metals — of  possibly  all  the  metals  which  we  know  upon 
this  earth.  If  we  could  remove  one  by  one  the  spec- 
trum of  these  metals,  one  could  obtain  the  spectrum  of 
the  glowing  furnace  beneath  the  atmosphere  made  by 
the  vapours.  There  seems  to  be  no  doubt  that  certain 
of  the  peculiar  bands  due  to  carbon  can  be  detected  in 
the  solar  spectrum.  They  are,  however,  almost  oblit- 
erated by  the  overlying  absorption  lines  of  other  metals, 
especially  by  the  lines  due  to  iron.  In  order  to  form 
an  idea  of  the  amount  of  iron  in  the  vapour  of  the  sun, 
this  amount  of  iron  having  an  important  bearing  upon 
our  ideas  of  the  electro-magnetic  condition  of  the  sun, 
I  have  endeavoured  to  ascertain  how  much  of  the  va- 
pour of  iron  in  conjunction  with  the  vapour  of  carbon 
would  obliterate  the  banded  spectrum  of  the  latter  in 
the  atmosphere  of  the  sun.  To  this  end  I  obtained 
carbon  terminals  containing  definite  proportions  of  iron 
and  carbon.  The  iron  reduced  by  hydrogen  was  dis- 
tributed uniformly  throughout  the  mass  of  carbon. 
Chemical  analysis  showed  that  the  mixture,  so  to  speak, 
was  homogeneous.  Specimens  taken  from  different 
portions  of  the  carbons  showed  in  the  carbons  which 
I  burned  in  the  electric  arc  twenty-eight  per  cent  of 
iron  and  seventy-two  per  cent  of  carbon. 

The  method  of  experimenting  was  as  follows : 
That  portion  of  the  spectrum  of  the  sun  which  con- 
tains traces  of  this  peculiar  carbon  band  lying  at  wave 
length  3883*7,  which  had  been  almost  obliterated  by 
the  accompanying  lines  of  absorption  of  other  metals, 
among  them  those  of  iron,  was  photographed.  The 
pure  carbon-banded  spectrum  was  photographed  on  the 
same  plate  immediately  below  the  solar  spectrum,  and 


300  WHAT  IS  ELECTRICITY? 

the  spectrum  of  the  mixture  of  iron  and  carbon  imme- 
diately below  this.  It  was  seen  that  from  twenty-eight 
to  thirty  per  cent  of  iron  in  combination  with  seventy- 
two  to  seventy  per  cent  of  carbon  almost  completely 
obliterated  the  peculiar  banded  spectrum  of  carbon. 
This  proportion,  therefore,  of  iron  in  the  atmosphere 
of  the  sun,  were  there  no  other  vapours  of  metals  pres- 
ent, would  be  sufficient  to  prevent  our  seeing  the  full 
spectrum  of  carbon. 

The  light  of  the  sun  and  that  of  the  electric  furnace 
closely  resemble  each  other.  The  light  of  the  electric 
furnace  is  due  to  the  combustion  of  carbon.  Can  we, 
therefore,  conclude  that  the  sun  is  a  vast  electric  fur- 
nace ? 

An  atmosphere  of  oxygen  greatly  augments  the 
vividness  of  the  latter.  The  question,  therefore,  whether 
oxygen  exists  in  the  sun  is  closely  related  to  questions 
in  regard  to  the  presence  of  carbon  when  we  consider 
the  temperature  and  light  of  the  sun. 

If  suppositions  also  are  made  in  regard  to  the  mag- 
netic condition  of  the  atmosphere  of  the  sun,  it  is  of 
great  interest  to  determine  whether  oxygen  exists  there ; 
for  oxygen  has  been  shown  by  Faraday,  and  later  by 
Prof.  Dewar,  to  be  strongly  magnetic. 

Prof.  Henry  Draper  brought  forward  evidence  to 
prove  the  existence  of  bright  oxygen  lines  in  the  solar 
spectrum.  Prof.  Hutchins,  of  Bowdoin  College,  and 
myself  examined  this  evidence,  and,  after  a  long  study 
of  the  oxygen  spectrum  in  comparison  with  the  solar 
spectrum,  came  to  the  conclusion  that  the  bright  lines  of 
oxygen  could  not  be  distinguished  in  the  solar  spectrum. 
Since  we  published  our  paper,  in  1885,  I  have  lately 
studied  the  subject  from  another  standpoint.  I  care- 
fully examined  the  regions  in  the  solar  spectrum  where 


THE  SUN.  301 

the  bright  lines  of  oxygen  should  occur  if  they  mani- 
fest themselves,  in  order  to  see  if  any  of  the  fine  ab- 
sorption lines  of  iron  in  the  spectrum  of  iron  were  ab- 
sent; for  it  is  reasonable  to  suppose  that  the  bright 
nebulous  lines  of  oxygen  would  obliterate  the  faintest 
lines  of  iron. 

The  method  adopted  by  Draper  for  obtaining  the 
spectrum  of  oxygen  consisted  in  the  employment  of 
a  powerful  spark  in  ordinary  air.  To  obtain  this  spark 
the  current  from  a  dynamo  running  through  the  pri- 
mary of  a  Kuhmkorff  coil  was  suitably  interrupted. 
By  the  use  of  an  alternating  machine  and  a  step-up 
transformer  suitable  sparks  can  be  more  readily  ob- 
tained, since  the  time  of  exposure  with  a  grating  of 
large  dispersion  is  long.  Considerable  heat  is  de- 
veloped in  the  transformer  from  the  powerful  cur- 
rents which  are  necessary  to  produce  a  spark  of  suffi- 
cient brilliancy.  I  have  therefore  modified  the  method 
in  the  following  manner :  The  spark  gap  is  inclosed 
in  a  suitable  chamber  which  can  be  exhausted ;  when 
the  exhaustion  is  pushed  to  a  certain  point  the  length 
of  the  spark  can  be  increased  ten  or  twelve  times  over 
its  length  in  air,  and  a  suitable  spark  for  photographic 
purposes  can  therefore  be  obtained  by  the  employment 
of  far  less  electrical  energy  in  the  transformer.  A 
pressure  of  eight  to  ten  inches  of  mercury  in  the  ex- 
hausted vessel  is  sufficient.  A  quartz  lens  inserted  in 
the  walls  of  the  exhausted  chamber  serves  to  focus 
the  light  of  the  spark  on  the  slit  of  the  spectroscope. 
A  careful  examination  of  the  solar  spectrum  showed 
that  none  of  even  the  finest  iron  lines  were  obliterated 
in  the  spaces  where  the  bright  oxygen  lines  should 
occur. 

Lord  Salisbury,  in   his  address  before  the  British 


302  WHAT  IS  ELECTRICITY! 

Association  at  Oxford,  1894,  remarks :  "  Oxygen  con- 
stitutes the  largest  portion  of  the  solid  and  liquid  sub- 
stances of  our  planet,  so  far  as  we  know  it ;  and  nitro- 
gen is  very  far  the  predominant  constituent  of  our 
atmosphere.  If  the  earth  is  a  detached  bit,  whirled 
off  the  mass,  leaving  the  sun,  we  cleaned  him  out  so 
completely  of  his  nitrogen  and  oxygen  that  not  a  trace 
of  these  gases  remain  behind  to  be  discovered  even  by 
the  sensitive  vision  of  the  spectroscope." 

Although  we  have  not  succeeded  in  detecting  oxy- 
gen in  the  sun,  it  seems  to  me  that  the  character  of 
its  light,  the  fact  of  the  combustion  of  carbon  in  its 
mass,  the  conditions  for  the  incandescence  of  the 
oxides  of  magnesium,  of  lanthanum,  and  of  the  other 
oxides  of  the  rare  earths  which  exist,  would  prevent  the 
detection  of  oxygen  in  its  uncombined  state.  Notwith- 
standing the  negative  evidence  which  I  have  brought 
forward,  I  can  not  help  feeling  strongly  that  oxygen 
is  present  in  the  sun,  and  that  the  sun's  light  is  due 
to  carbon  burning  in  an  atmosphere  of  oxygen. 

Can  not  also  the  dark  spots  on  the  sun  be  explained 
by  Kirchhoff  and  Stefan's  law,  that  in  a  heated  space  a 
bundle  of  rays  made  up  of  direct  and  reflected  rays  from 
a  surface  show  the  same  peculiarities  that  a  bundle  of 
rays  from  a  dark  hot  body  would  show  ?  A  dark  spot 
on  the  sun  is  merely  a  hole  in  the  gaseous  envelope 
through  which  we  look  into  an  oven.  The  direct  and 
reflected  rays  with  the  supposable  layers  of  vapours  of 
different  light-emission  power  make  the  interior  of  the 
oven  of  varying  light  intensity.  From  the  balancing 
of  the  reflected  and  direct  rays,  a  smaller  amount  of 
rays  reach  the  eye  from  certain  vapours  than  from  the 
outer  envelope  on  which  this  balancing  of  direct  and 
reflected  rays  does  not  take  place. 


THE  SUN.  303 

Kirchhoff  and  Stefan's  law  has  lately  received  a 
great  deal  of  attention  from  experimenters  in  Germany. 
W.  Wien  and  O.  Lummer  suggest  that  two  pieces  of 
thin  platinum  foil,  brought  to  incandescence  by  an 
electric  current,  be  placed  opposite  each  other  in  a  fur- 
naoe.  One  is  provided  with  a  slit  through  which  the 
other  can  be  viewed.  The  inner  appears  much  bright- 
er than  the  outer,  since  it  is  shielded  from  the  reflec- 
tion of  the  walls  of  the  oven  by  the  outer  foil.  The 
temperature  of  the  pieces  of  foil  can  be  determined  by 
the  increase  of  resistance  of  the  platinum.  The  ar- 
rangement can  be  thus  used  as  a  bolometer,  the  radiation 
to  be  measured  being  sent  through  the  slit,  and  both 
pieces  of  platinum  being  thus  heated.  In  this  way  a 
result  is  obtained  which  is  independent  of  the  indi- 
vidual peculiarities  of  the  absorbing  and  emitting  sur- 
faces, and  the  absolute  radiation  can  be  measured  more 
correctly  than  by  previous  methods.  Mr.  St.  John, 
late  holder  of  the  Tyndall  scholarship  of  Harvard  Uni- 
versity, working  with  Prof.  "Warburg  in  Berlin,  also 
independently  of  Wien  and  Lummer,  worked  out  a 
practical  method  of  measuring  the  light  emitted  by 
various  substances  at  high  temperatures.  His  method 
suggests  the  application  of  Kirchhoffs  law  to  the  ques- 
tion of  the  light  of  the  dark  spots  on  the  sun.  Two 
pieces  of  platinum  foil  were  suspended  side  by  side  in 
an  oven  which  was  heated  to  a  high  temperature.  Ob- 
servations of  their  incandescence  were  made  through  a 
suitably  placed  window  by  means  of  a  photometer. 
When  one  piece  of  foil  was  coated  with  an  oxide  of  the 
rare  earth's  zirconium,  lanthanum,  etc.,  it  appeared 
brighter  than  the  uncoated  piece.  If  one  piece  of  foil 
was  inclined  to  the  wall  of  the  oven  so  that  the  re- 
flected rays  from  the  walls  of  the  oven  were  sent 


304  WHAT  IS  ELECTRICITY? 

through  the  window,  the  two  pieces  of  foil  could  be 
made  to  appear  of  the  same  intensity.  The  sum  of  the 
direct  and  reflected  light  is  then  equal  for  both  pieces 
of  foil.  The  uncoated  piece  must  reflect  just  as  much 
more  light  than  the  coated  as  it  is  deficient  in  the 
amount  of  direct  light  it  can  transmit.  This  is  in  ac- 
cordance with  Kirchhoff's  law — that  is,  that  hi  a  heated 
space  a  bundle  of  rays  made  up  of  direct  and  reflected 
rays  from  a  surface  show  the  same  peculiarities  that  a 
bundle  of  rays  from  a  dark  hot  body  would  show.  Mr. 
St.  John  utilized  this  idea  by  bringing  a  cold  porcelain 
cylinder  into  the  neighbourhood  of  the  pieces  of  foil ; 
the  bare  platinum  could  then  be  quickly  distinguished 
from  the  surrounding  hot  walls,  and  appeared  darker 
than  the  coated  platinum.  As  soon  as  the  rod  took  the 
temperature  of  the  oven  the  field  of  view  appeared  uni- 
formly bright. 

Such  are  some  of  the  considerations  to  which  I  am 
led  by  a  study  of  the  electric  oven ;  they  must  be  re- 
garded not  as  final  considerations,  but  merely  as  at- 
tempts to  penetrate  into  the  mysteries  of  the  sun. 


CHAPTEE  XXII. 

WHAT   IS    ELECTRICITY? 

THE  old  fluid  theories  of  electricity  can  be  said  to 
have  been  buried  in  a  common  grave  with  the  theories 
of  caloric  and  phlogiston,  and  Faraday's  researches  give 
the  deathblow  to  the  old  theory  of  action  at  a  distance 
(J.  J.  Thomson).  I  have  endeavoured  to  show  in  the 
preceding  chapters  that  in  the  phenomena  of  the  trans- 
formations of  energy  there  is  a  great  fieJd  of  investiga- 
tion which  will  repay  the  student  to  study  and  to  work 
in.  If  we  can  not  discover  what  electricity  is,  we  can 
ascertain  its  relations  to  light  and  heat.  The  human 
mind,  however,  is  far-reaching,  and  loves  to  frame  hy- 
potheses and  theories,  and  perhaps  no  subject  is  fuller 
of  theories  than  that  of  electricity. 

If  we  abandon  Maxwell's  electro-magnetic  theory 
of  light,  we  find  that  we  must  choose  between  a  great 
number  of  rival  theories — the  theories  of  Ampere, 
Grassman,  Stefan,  Korteweg,  Neumann,  Gauss,  Keber, 
Kiemann,  and  Clausius — in  which  the  actions  in  a  me- 
dium between  magnets  and  electrical  circuits  are  not 
considered.  Moreover,  the  various  rotational  phenom- 
ena of  magnetism  and  electricity  are  not  fully  embraced 
in  these  theories.  These  theories,  moreover,  do  not 
consistently  connect  the  manifestations  of  light,  heat, 
and  electricity,  and  most  of  them  are  not  founded  on 


306  WHAT  IS  ELECTRICITY? 

the  doctrine  of  the  conservation  of  energy.  Prof.  J.  J. 
Thomson,  in  a  report  on  electrical  theories,*  divides  the 
theories  into  the  following  classes : 

"  1.  Theories  in  which  the  action  between  elements 
of  currents  is  deduced  by  geometrical  considerations, 
combined  with  assumptions  which  are  not  explicitly,  at 
any  rate,  founded  on  the  principle  of  the  conservation 
of  energy.  This  class  includes  the  theories  of  Ampere, 
Grassman,  Stefan,  and  Korteweg. 

"2.  Theories  which  explain  the  action  of  currents 
by  assuming  that  the  forces  between  electrified  bodies 
depend  upon  the  velocities  and  accelerations  of  the 
bodies.  This  class  includes  the  theories  of  Gauss,  Weber, 
Kiemann,  and  Clausius. 

"  3.  Theories  which  are  based  upon  dynamical  con- 
siderations, but  which  neglect  the  action  of  the  dielec- 
tric. This  class  contains  L.  E.  Neumann's  potential 
theory  and  Helmholtz's  extension  of  it. 

"  4.  C.  Neumann's  theory. 

"  5.  Theories  which  are  based  upon  dynamical  con- 
siderations, and  which  take  into  account  the  action  of 
the  dielectric.  This  class  includes  the  theories  of  Max- 
well and  Helmholtz." 

Prof.  Thomson  criticises  the  various  theories,  and 
shows  "  that  they  can  be  divided  into  two  great  classes, 
according  as  they  do  or  do  not  take  into  account  the 
action  of  the  dielectric  surrounding  the  various  conduc- 
tors in  the  field."  According  to  the  potential  theories 
of  L.  E.  Neumann  and  Helmholtz,  in  an  unclosed  cir- 
cuit there  are  forces  which  arise  from  the  discontin- 
uity at  the  ends  of  the  circuit.  Shiller's  f  experiments 

*  Report  of  the  British  Association,  Aberdeen,  1885. 
f  Poggendorf  s  Annalen,  vol.  clix,  p.  456. 


WHAT  IS  ELECTRICITY?  307 

show,  however,  that  this  potential  theory  is  wrong  "  if 
we  neglect  the  action  of  the  dielectric  and  assume  that 
the  current  stops  at  the  end  of  the  circuit."  Shiller 
also  showed*  that  Ampere's  theory  fails  for  open  cir- 
cuits, and  that  Grassman's  and  Clausius's  theories  must 
be  wrong  as  well  as  Ampere's  and  Korteweg's,  for 
Shiller's  experiments  proved  that  the  dielectric  must 
be  taken  into  account. 

Maxwell's  great  theory  is  full  of  the  intimations 
which  we  all  have  in  regard  to  the  probable  relation 
between  the  varied  transformations  of  energy.  It  rests, 
however,  upon  the  hypothesis  of  the  existence  of  dis- 
placement currents  in  a  non-conductor,  and  the  exist- 
ence of  these  currents  has  never  been  satisfactorily 
shown  by  experiment. 

When  a  Leyden  jar,  for  instance,  is  discharged  by 
connecting  the  outer  coating  to  the  inner  by  a  wire,  it 
is  supposed  by  Maxwell  that  displacement  currents  of 
electricity  occur  in  the  glass  of  the  jar,  which  is  the 
dielectric  which  separates  the  outer  tin-foil  coating  of 
the  jar  from  the  inner  coating.  These  currents  are 
called  displacement  currents,  to  distinguish  them  from 
the  apparent  current  in  the  wire  which  discharges  the 
jar.  The  electrical  state  on  the  coatings  of  the  jar  is 
supposed  to  be  rapidly  displaced  to  and  fro  by  the  os- 
cillations of  the  jar.  The  conduction  currents  in  the 
wire  are  transformed  into  heat.  "While  the  displace- 
ment currents  do  not  manifest  this  transformation,  they 
are  supposed,  however,  to  exert  magnetic  influences  like 
ordinary  conduction  currents.  They  not  only  are  called 
into  existence  in  the  glass  or  dielectric  of  the  jar,  bu* 
they  also  appear  in  the  air  surrounding  the  jar.  They 


*  Poggendorf  s  Annalen,  vol.  clix.  p.  456. 

21 


308  WHAT  IS  ELECTRICITY? 

are  instantaneous  currents,  and  depend  on  the  rate  of 
change  of  the  electro-motive  force,  or  difference  of  po- 
tential between  the  coatings  of  the  Leyden  jar,  and  also 
upon  the  substance  of  the  dielectric,  whether  it  be 
glass,  or  rubber,  or  air. 

I  have  said  that  the  existence  of  these  displacement 
currents  has  never  been  proved  by  experiments  which 
are  free  from  criticism.  The  most  satisfactory  investi- 
gation is  that  of  Hertz,  who  apparently  showed  that 
displacement  currents  in  a  large  pile  of  books,  and  in 
large  masses  of  other  dielectrics,  exerted  a  magnetic 
effect  upon  the  electric  waves  emanating  from  an  oscil- 
lator. The  position  of  the  little  circle  of  wire  (N,  Fig. 
47)  constituting  his  exploring  resonator  was  influenced 
by  the  proximity  of  a  large  solid  dielectric,  such  as  a 
block  of  paraffin,  and  he  attributed  the  disturbance  to 
Maxwell's  displacement  currents  in  this  dielectric.  Prof. 
J.  J.  Thomson  remarks :  *  "  The  most  pressing  need  in 
the  theory  of  electro-dynamics  seems  to  be  an  experi- 
mental investigation  of  the  question  of  the  continuity 
of  these  dielectric  currents.  We  have  experimental 
proof  that  they  exist  (?),  but  we  do  not  know  whether 
Maxwell's  assumption  that  they  always  form  closed  cir- 
cuits with  the  other  currents  is  true  or  not.  If  Max- 
well's assumption  should  turn  out  to  be  true,  we  should 
have  a  complete  theory  of  electrical  action." 

What  shall  we  therefore  answer  to  the  question, 
What  is  electricity  ?  Must  we  reply,  Ignoramus  igno- 
rabimus — (We  are  ignorant,  and  we  shall  remain  igno- 
rant) ?  We  have  already  strong  grounds  for  believ- 
ing that  we  live  in  a  medium  which  conveys  to-and- 
fro  or  periodic  movements  to  us  from  the  sun,  and 

*  Report  on  Electrical  Theories. 


WHAT  IS  ELECTRICITY?  309 

that  these  movements  are  electro-magnetic,  and  that 
all  the  transformations  of  light  and  heat,  and  indeed 
the  phenomena  of  life,  are  due  to  the  electrical  energy 
which  comes  to  us  across  the  vacuum  which  exists  be- 
tween us  and  the  sun — a  vacuum  which  is  pervaded  by 
the  ether,  and  which  is  a  fit  medium  for  the  transmis- 
sion of  the  electro-magnetic  waves. 


INDEX. 


Absolute  measurements,  11. 

Air  and  gravitation,  16. 

Air,  compressed,  47. 

Air,  telegraphing  through,  225. 

Alloys,  81. 

Alternating  currents,  133, 141, 154. 

Alternating  machine,  139. 

American    Academy    of  Arts    and 

Sciences,  60. 
Ampere,  62. 

Anomalous  magnetism,  185. 
Arago,  159. 

Armstrong  electrical  machine,  212. 
Arons,  Prof.,  234. 
Atlantic  cable,  171. 
Atmospheric  electricity,  212. 
Aurora,  213. 

Battery,  storage,  74, 164 ;  and  bicycle, 

75. 

Bell,  Graham,  237. 
Bjerknes,  Prof.  C.  A.,  42. 
Bjerknes,  V.,  257. 
Bolometer,  233. 
Boussinesq,  297. 
Boys,  C.  V.,  15,  96,  216. 
Bridge,  Wheatstones,  234. 
Brush  discharge,  208. 

Cailletet,  M.,  114. 

Carbon  and  iron  in  the  sun,  299. 

Cathode,  281. 

Cathode-ray  lamp,  196. 


Cathode  rays,  284. 

Cautery,  electric,  53. 

Cell,  Latimer  Clark,  93. 

Cell,  voltaic,  64. 

Challis,  Prof.,  21. 

Circuit,  earth,  57. 

Circuit,  magnetic,  39, 109. 

Circuit,  multiple,  55. 

Coal  and  electricity,  79. 

Coherent  tubes,  253. 

Coils,  Euhmkorff,  188. 

Coladeau,  M.,  114. 

Cold,  magnetism  and,  33. 

Colour  photography,  228. 

Commutator,  99,  123. 

Compressed  air,  47. 

Compressed-air  motors,  116. 

Conductors  and  insulators,  84. 

Consequent  poles,  36. 

Constantan,  78. 

Copper  and  glass,  191. 

Copper,  purity  of,  53. 

Copper,  resistance  of,  53. 

Crookes  tubes,  193. 

Current,  electric,  45. 

Currents,  alternating,  133, 141, 154. 

Currents,  displacement,  307. 

Currents,  high-frequency,  190. 

Currents,  to-and-fro,  127. 


Decay  of  electric  energy  in  copper, 
134. 


Gil 


312 


WHAT  IS  ELECTRICITY? 


De  la  Kive,  113. 
Depretz,  M.,  152. 
Dewar,  Prof.,  34. 
Diffraction  gratings,  230. 
Dimensions  of  waves  of  light,  223. 
Dip  storage  battery,  114. 
Discharge  of  a  Leyden  jar,  172. 
Discharge,  oscillatory,  184. 
Displacement  currents,  307. 
Dolbear,  Prof.,  236. 
Draper,  Prof.  Henry,  300. 
Dynamo  machine,  94 ;  efficiency  of, 

138. 
Dyne,  15. 

Earth  circuit,  57. 

Earth,  flow  of  electricity  in  the,  57. 

Ebert,  195. 

Edison  lamp,  172,  210. 

Efficiency  of  dynamo,  1 38. 

Efficiency  of  electric  light,  194. 

Electrical  machine,  178 ;  Armstrong, 
212. 

Electrical  theories,  305. 

Electric  cautery,  53. 

Electric  current,  45. 

Electric  energy  in  copper,  decay  of, 
134. 

Electric  furnace,  83. 

Electricity  and  steam,  104. 

Electricity,  atmospheric,  212;  fluid 
theories,  305 ;  from  coal,  79 ;  heat- 
ing by,  54, 118 ;  velocity  of,  249 ; 
frictional,  76 ;  velocity  of,  165,  260; 
transmission  of  power  by,  109, 139 ; 
quantity  of,  12. 

Electric  light,  efficiency  of,  194. 

Electric  motors,  107. 

Electric  power,  sources  of,  105. 

Electric  pressure,  138. 

Electric  spark,  energy  of,  268 ;  per- 
forations, 202 ;  photographing,  204 ; 
energy  in,  175. 

Electric  stress,  269. 

Electric  waves,  239  ;  in  air,  249 ;  and 
vegetation,  117 ;  and  photography, 
120  ;  in  the  sun,  267. 


Electric  welding,  83, 157. 
Electric  wires  and  gas  pipes,  176. 
Electrolysis,  72. 

Electrolysis  and  plant  growth,  117. 
Electrolytic  action  and  underground 

pipes,  74. 

Electrolytic  polarization,  73. 
Electro-magnetic    theory   of    light, 

264,  273. 

Electrometer,  92. 
Electro-motive  force,  130. 
Electrostatic  force,  line  of,  189.J 
Electrotonic  state,  128. 
Energy  of  lightning,  176. 
Energy,     transformations   of,    121 ; 

electric  spark,  175,  268. 
Engines,  oil,  116 ;  men,  considered  as, 

118;  hot-air,  116;  plants  as,  119. 
Ether,  273 ;   and  movements  of  the 

earth,  231. 

Farad,  63. 

Faraday,  9, 122, 148, 172 ;  on  gravita- 
tion, 21. 

Feddersen,  183. 

Fcrnkrafte,  276. 

Ferraris,  Prof.,  159. 

Fibre,  quartz,  97. 

Field,  magnetic,  130. 

Fireflies,  197. 

Flame,  manometric,  221. 

Flow  of  electricity  in  the  earth, 
57. 

Fluid  theories  of  electricity,  305. 

Fluid  theory,  Franklin's,  199. 

Fluoroscope,  292. 

Focus  tubes,  291. 

Force,  electro-motive,  130  ;  magneto- 
motive force,  40. 

Frankfort,  160. 

Franklin,  Benjamin,  26,  31,  64, 170 ; 
fluid  theory  of,  199. 

Frictional  electricity,  76. 

Furnace,  electric,  83. 

Galvani,  65. 

Galvanometer,  the,  86 ;  marine,  132 ; 


INDEX. 


313 


microscope  and,  88 ;    minor,    95 ; 

needle,  95. 
Gas  engines,  115. 
Gas  pipes  and  electric  wires,  176. 
Glass  and  copper,  191. 
Goldhammer,  D.  A.,  296. 
Gratings,  diffraction,  230;  concave, 

231. 
Gravitation,  Faraday    on,  21;   and 

the  air,  16  ;  and  the  ether,  16, 18, 

19. 
Grove  cell,  70. 

Harvard  LTniversity,  Jeflerson  Phys- 
ical Laboratory  of,  50. 

Harvey,  87. 

Heat,  mechanical  equivalent  of,  12. 

Heat  waves  and  electric  waves,  266. 

Heating  by  electricity,  54, 118. 

Helmholtz,  293. 

Henry,  Joseph,  9,  47,  121,  182,  185, 
239,  262. 

Henry,  the,  63. 

Herschel,  Sir  John,  225. 

Hertz,  249,  267. 

High  electrical  pressure,  154. 

High-frequency  currents,  190. 

Holtz  machine,  179. 

Horse  power  in  lightning,  177. 

Hot-air  engines,  116. 

Hutchins,  Prof.,  300. 

Incandescence,  55. 

Induction,  135  ;  magneto-,  124 ;  self-, 

164. 

Insulated  wire,  45. 
Insulators,  conductors  and,  84. 
Interference  apparatus,  230. 
Iron,  resistance  of,  53. 

Jaumann,  293. 

Joule,  118. 

Junctions,  thermal,  151. 

Kelvin,  Lord,  14,  19,  21,  37,  44,  90, 

240,  275. 
Kirchhoff,  302. 


Klemencic,  255. 
Kohlrausch,  K.,  82. 

Labile  ether,  275. 

Lamps  in  parallel,  55 ;  Edison,  210. 

Langley,  Prof.,  75, 196,  233. 

Latimer  Clark  cell,  93. 

Lauffen,  160. 

Leclanche  cell,  70. 

Lenard,  284. 

Length,  standards  of,  11. 

Le  Sage,  20. 

Leuwenhoek,  87. 

Leyden  jar,  155, 171,  240 ;  discharge 

of  a,  172. 

Light  and  magnetism,  271. 
Light,    electro-magnetic   theory  of, 

264,  273. 

Light,  polarization  of,  80. 
Light,  undulatory  theory,  objections 

to,  280. 
Lightning,  155, 198 ;  horse  power  in, 

177 ;   energy  of,  176 ;  oscillatory 

nature  of,  207  ;  potential  of,  206. 
Lines   of    electrostatic    force,    189 ; 

magnetic  force,  127. 
Lippman,  228. 
Listening  under  water,  222. 
Lodge,  Prof.,  166,  252,  267. 
Long-distance  telephony,  160. 
Longitudinal  waves,  222. 
Lummer,  303. 

Machine,  alternating,  139  ;  dynamo, 
94;  Holtz,  179;  electrical,  178; 
man  considered  as  a,  118. 

Magnetic  circuit,  39, 109 ;  field,  130 ; 
induction,  124, 135 ;  lines  of  force, 
127;  shields,  131;  rotary,  field, 
142, 159. 

Magnetism  and  cold,  33 ;  anomalous, 
185 ;  and  light,  271 ;  and  a  medi- 
um, 38. 

Magneto-motive  force,  40. 

Manometer  flame,  221. 

Marey,  119. 

Marine  galvanometer,  132. 


314: 


WHAT  IS  ELECTRICITY? 


Maxim,  116. 

Maxwell,  J.  C.,  4,  41, 122, 148,  271. 

Measurements,  absolute,  11. 

Mechanical  equivalent  of  heat,  12. 

Medium,  magnetism  and  a,  38; 
stress  and  the,  41 ;  storing  of  ener- 
gy in,  270. 

Michelson,  Prof.,  230. 

Microscope  and  galvanometer,  88. 

Mirrors,  parabolic,  251 ;  revolving, 
184 ;  galvanometer,  96. 

Molecular  vortices,  281. 

Motors,  compressed-air,  116;  elec- 
tric, 107. 

Movement  of  the  earth  and  the  ether, 
231. 

Multiple  circuit,  55. 

Nahekrafte,  276. 

Needle,  galvanometer,  95. 

Niagara  Falls,  transmission  of  power 

from,  111,  152, 161. 
Nicol's  prisms,  80. 
Nobert,  229. 

Objections  to  the  undulatory  theory 

of  light,  280. 
Odic  force,  40. 
Ohm,  63. 
Oil  engines,  116. 
Oscillatory  discharge,  184. 
Oscillatory  nature  of  lightning,  207. 
Oxygen,  129 ;  in  the  sun,  300. 

Parabolic  mirrors,  251. 

Perforation,  electric-spark,  202. 

Phase,  143. 

Photography  of  air  waves,  216 ;  col- 
our, 228;  electric  waves,  120; 
electric  sparks,  204. 

Photophone,  237. 

Physical  Laboratory,  Jefferson,  ot 
Harvard  University,  50. 

Plant  growth  and  electrolysis,  117. 

Plants  as  engines,  119. 

Polarization,  electrolytic,  73 ;  of 
light,  80. 


Poles,  consequent,  36. 
Potential  of  lightning,  206. 
Poynting,  Prof.,  13,  84, 166, 191. 
Preece,  Mr.,  60. 

Pressure,  high  electrical,  154 ;  elec- 
tric, 138. 

Prisms,  Nicol's,  80. 
Purity  of  copper,  53. 

Quantity  of  electricity,  12. 
Quartz  fibres,  97. 

Rayleigh,  Lord,  213. 
Reflection  of  X  rays,  296. 
Refraction  of  electric  waves,  253 ;  of 

X  rays,  296. 

Reichenbach,  Baron,  40. 
Resistance  of  copper.  53 ;  of  iron,  53. 
Resonance,  240. 
Revolving  mirror,  184. 
Reymond,  du  Bois-,  277. 
Rhigi,  Prof.,  253,  296. 
Roberts,  Prof.  Austen,  81. 
Rontgen,  288. 

Rotary  magnetic  field,  142,  159. 
Rowland  concave  grating,  231. 
Rubens,  Prof.,  234. 
Ruhmkorff  coil,  140, 188. 
Rumford,  Count,  7,  26,  28,  111. 

St.  John,  Mr.,  303. 

Salisbury,  Lord,  2,  273,  302. 

Self-induction,  164. 

Sellineyer,  297. 

Shields,  magnetic,  131. 

Siemens,  Sir  William,  152, 157. 

Skin  effect,  190. 

Solar  spectrum,  298. 

Somerville,  Mrs.,  271. 

Sources  of  electric  power,  105. 

Spectroscope,  232. 

Spottiswoode,  204. 

Standard  of  length,  11 ;  of  time,  11. 

Stationary  waves,  226. 

Steam  and  electricity,  104. 

Stefan,  302. 

Step-down  transformers,  156. 


INDEX. 


315 


Step-up  transformers,  155, 187. 
Storage  battery,  74,  112;  dip,  114, 

162 ;  zinc-lead,  113. 
Storing  of  energy  in  a  medium,  270. 
Stress  and  the  medium,  41 ;  electric, 

269. 

Submarine  telegraphy,  180. 
Sun,  the,  298;  carbon  and  iron  in, 

299 ;  oxygen  in,  300 ;  electric  waves 

on,  267. 

Tait,  Prof.,  216. 

Telegraphing    without    wires,    60; 

submarine,  180 ;  through  the  air, 

225. 
Telephone,  124,  144;  long-distance, 

160. 

Teala,  187, 192. 
Theories,  electrical,  305. 
Thermal  junctions,  151. 
Thermo-electricity,  77. 
Thomson,  Elihu,  146,  147,  157, 187, 

291. 

Thomson,  J.  J.,  81,  267,  282,  295. 
Time,  standards  of,  11. 
To-and-fro  currents,  127. 
Topler,  219. 

Transformations  of  energy,  121. 
Transformers,  155, 156, 187. 
Transmission  of  power  by  electricity, 

109,  111,  139 ;  from  Niagara  Falls, 

152,161. 
Tyndall,  Prof.,  5,  70,  78, 110. 

Underground  pipes  and  electrolytic 
action,  74. 


Vegetation  and  electric  waves,  117. 
Velocity    of    electricity,    165,    249, 

260. 

Velocity  of  sound  waves,  222. 
Visibility  of  sound  waves  in  the  air, 

219. 

Volt,  62. 
Volta,  69. 
Voltage,  62. 
Voltaic  arc,  82. 
Voltaic  cell,  64. 
Vortices,  molecular,  281. 

Warburg,  Prof.,  302. 

Water,  listening  under,  222. 

Waves,  stationary,  226 ;  velocity  of 
sound,  222 :  study  of  sound,  221 ; 
photography  of  air,  216;  dimen- 
sions of  light,  223;  electric  and 
heat,  266 ;  electric,  in  air,  249 ;  re- 
fraction of  electric,  253 ;  visibility 
of  sound,  in  air,  219 ;  longitudinal, 
222. 

Weber,  W.,  82. 

Welding,  electric,  83, 157. 

Wheatstones  Bridge,  234. 

Wien,  W.,  303. 

Wiener,  226. 

Winthrop,  Prof.  John,  26, 170. 

Wire,  insulated,  45. 

Wires,  telegraphing  without,  60. 

X  rays,  287;  refraction  and  reflec- 
tion of,  296. 

Zinc-lead  storage  cells,  113. 


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